As the title says, this is about stats and Ordo Sicarius Operatives.
Mostly - what should they be?
The classic question, so I'll give the classic answer: the stats should be appropriate for whatever you character you create. Flesh out the character first, then the stats will follow (don't worry, I have more helpful advice below).
I read the Recongregator sourcebook, and I'm now tempted to create an Amalathian Inquisitor who has one (maybe two) Assassinorum trained Operatives
Judging by the modeling forum, you're well on your way, great!
But the thought of a statline for such an operative concerned me - the classic rulebook suggests things like 100 Initiative for an Assassin, which is quite insane in terms of board-balanced stats relative to the descriptions provided for what humans are capable of.
Ignore the rulebook and readjust your thinking to your gaming group. If you don't have a specific gaming group, you're in luck because you get to start the baseline. Otherwise, if you're attending a Conclave event in the UK I'll just point you back to those gentleman (for point of reference, check out their own characters posted on this board).
Which begs the question, what would be suitable or expected for such a character?
Who would they be?
How would they act or be played?
What would make them interesting?
So here's the real meat of it (and the more interesting part of the question). There's a number of different ways to balance these characters. Off the bat, give them a high Initiative. 100 (or close to that) isn't necessarily insane depending on the situation. Arguably the most important characteristic the assassin will need to have is their reaction speed. Whether they're a fighter, a spy who infiltrates, a sniper, whatever, they're going to need to be able to mentally assess a situation quickly and then physically react to it just as fast. 100 is on the high end (technically speaking 100 should be the cap of human evolution, sans augmentation, but that'll depend on your gaming group).
Moving on from that, the rest will depend. A sniper's WS should be high. 70s is high for some group, 90s for others. That said, I'd encourage you to set up a board and place some minis just to test yours out. The right (or wrong) combination of WS and gun can result in a character who takes on aim action and is basically guaranteed to hit anything on a board within LOS (given that most of us aren't playing on 8x8 boards). So temper the stats so your character is actually fun to play.
In the case of a sniper, you'll probably want them to be a competent fighter, but not overly so. 50s to 70s (again, depending on the group) would work. Reverse all that for melee assassin.
Now comes Strength and Toughness. You don't need your assassin to be crazily high on these stats (unless that's how they're conditioned). Sure they're in good shape, but in many cases they could be expected to kill without detection or from afar. While they'll definitely be strong and tough they're not Space Marines or body builders. It's all based on utility. You could even have some rules to enhances aspects of their abilities relative to their stats (a bonus to jumping for instance that isn't tied to strength. apologies if I'm misremembering that S is linked to jumping distance).
Anyway, S and T could be anywhere between 40s (for a nebbish spy infiltrator, which is probably lower than you'd want) up to 90s (if you're looking for a roided killer, which could be way too high).
I'd tend to go mid-range on SG (30s too 60s) but depending on the circumstance you could give bonuses based on mental stats if they need to pretend they know more about something than they do in casual conversation. They can convince ppl they're smart without really knowing what they're talking about (again, I'm thinking about the infiltrator). NV and LD would likely be at the higher end, depending. You wouldn't want someone used to taking out the random odd rebel leader to stare down a greater daemon, so again you could do anything from the 60s up to the 90s depending.
I think I've covered most of it with hypothetical rambling, but that's what I meant by "it depends" earlier. You could have an assassin that is really high in every regard, but in that case you'd have to balance them out with effective role playing. A character that is always taking steroid, at the gym, sparring, and cleaning their gun in between range time is not going to be very good with people. Think about your character from that angle, what do they do in their free time? If they have no free time, what is their training regimen? Are they studying targets and getting into character for an undercover mission?
I'll close by also suggesting you look at wargear. Someone who is super talented in every way may be more apt to be sent in undercover to do thing quietly (why would someone so highly skilled be sent on suicide missions?) which means they're going for subtlety. Having someone with a high BS and WS working exclusively with hidden daggers and a digi las pistol with limited shots dressed in Imperial formal-wear who is going up against a well armed group of mercenaries is still going to have a tough time. Even a high T won't save you from a dozen autogun rounds. Just saying, there are lot of ways to balance, so make what you like and let the GM sort it out
That was tons of tangents, but I hope that helps (in a very indirect way though it may be).