As I never updated this post, am doing it now since I just used this gun extensively today.
On single-shot mode its doing around 1.8-1.9j on 0.25g BBs. Tri-shot can't be chronoed as its a single-barrel system, but test fired and wasn't overly powerful (same amount of co2 is used for either mode).
Some general observations of the Cybergun FN SLP I had after playing:
-It seems like due to the feeding system and/or the fact that all 3 BBs are shot through one barrel, the hop-up doesn't really work too well when using the tri-shot function, and if applied too much for heavier BBs it won't distribute the same power across all 3 BBs, and eventually jam/stop feeding with too much hop applied. However, the hop-up has no issues when using the single-shot mode.
-Range is not anything insane compared to a proper DMR-power gun, probably due to the weight of BB I was using. Accuracy is good but some occasional random shots would veer off to the side, however at 20-40m where I was it was very usable, was essentially playing it as a DMR with no scope.
-10/10 impressive sound, makes a hugely satisfying crack with every shot, people know you're shooting at them and they hate it.
-No need for a sidearm as switching to the 3 shot mode is essentially instant and immediately lowers the power to around 1j (as stated by Cybergun), and even with funky hopping on 0.25s & 0.28s which I used, still perfect for within 20m. Above 20m used the single-shot mode.
-Insanely fun in CQB, I only used it for a short bit and tuned down the hop up massively so I could put it in the tri-shot mode, but the sound of the gun, combined with the fact it actually looks like a shotgun (sorry AA-12 & SGR12) while having semi-auto, and shooting 3 BBs per shot, is just so much fun to use. The BBs are grouped fairly close until 20m which is a decent range, so I'm excited to take it to a proper full CQB site to try it out there with the tri-shot mode only.
Final thoughts on the gun:
Its a shame it seems to be intended for only US market as its a really interesting system, a kind of Mk23-like NBB but with a tri-shot mode, for only £140 as well.
I don't think it outperforms an AEG in any manner in the single-shot mode but its not the point of the gun. I love the fact its a tri-shot with semi-auto in a more normal looking shotgun body, cant wait to try in a full CQB site.