@proffrink is our resident HPA expert.
There might have been a time when that was true, but probably not anymore I'm afraid. I'm a bit out of the loop on the latest goings-on.
However for this question: SDiK is quite nice, Bolt is marginally more competitive (shorter, lighter bolt cycling). I would go with the SDiK if you don't mind the fact that it shoots a little slower (is this even a problem for such a gun?) and a tiny bit louder. Just make sure you use good quality pneumatic line and not some thin-walled stuff for fish tanks, or it will explode (not dangerously, but annoyingly). I've shot the Bolt and owned two of the SDiK. Both will accept a CO2 bulb (if that's your jam later on), but the Bolt needs space for a battery (not a big deal in a VSR, but check how much room you have in the stock for a CO2 adapter and battery if you're using the M40. I'm sure a million people have already done this.
The Mancraft HRR looks great (again if you want that lightweight CO2) but I've never touched one.
Can order directly from Mancraft if you want to by the way.
I agree with
@EvilMonkee though - you will be stripping out a major part of a nice gun. This is a better operation to do on a cheap body, then upgrade the hop and barrel accordingly (consistent power source, good hop unit and a nice barrel finish are the holy trinity of getting a really nice firing airsoft gun of any kind, but so much more so in a sniper rifle). If you can find a donor gun for a reasonably amount of money, you should do that and even consider selling the TM. They are nice guns out of the box, but if you're going to the effort of moving to HPA and really want the best of the best, even a well adjusted TM hop and barrel probably won't do that in a sniper rifle.
All of this is eclipsed by the fact that if this is your first BASR, you should get used to using it normally for a few months or a year first anyway. I don't know how much you've played so I might be stating the obvious but sniping gameplay is a lot different to anything else and you will have to do a lot of your own fiddling around with any sniper rifle to get it shooting in that top percentile (really the only justification for stripping it out and going with HPA in the first place). If you're not up for replacing the hop unit, barrel (and spacers, maybe a trigger etc.) it's not worth doing. Changing the power source alone won't get it shooting much better.