First your weights are exactly what I have found to be the absolute best for most common .22 & .25 airgun twists.
I don't know what your FX liner is but my 1:17.7 (alleged) twist can send 28-29's up to 1075. I use the 32's .22 and be it 900 or 1040, they stabilize and work.
I'm not Hunting. I don't care about shot count other than obvious overkill tune.
I shoot tethered with a step down in-line reg from Alkin (& have a big Alkin W31 3.5-4.7cfm compressor. Air is not a concern now.
Since I get the best results at 2500-2800psi I simply fill my SCBA to 3500psi instead of 4500psi. Easier on my regs high side, 16 year old SCBA, & the compressor gets run more often at lower load. All better.
Now to answer your questions I will say I have sent 32 grain RBT slugs from a .25 EDgun Supermagnum @ 940fps 316 yards. I've sent 32-55 grain .25's from a no choke .25 Condor & a Sumatra .25
The Sumatra sent 46 grain RBT slugs at 940. Just by chance. The Condor & EDgun Supermagnum could have been turned way up.
If you are just target shooting like me for groups at long range in winds or not. Why bother going heavier. Send the 28's as fast as possible for less flight time.
But FIRST, figure out what slug is optimal in your liner.
Optimal being you will need to know exact twist rate. Exact length of slug in thousandths. Exact OD is .250". Speed & atmospheric conditions.
http://www.jbmballistics.com/cgi-bin/jbmstab-5.1.cgi And I don't own any new FX guns. Just 2 classic ones.
I wouldn't want a FX platform to shoot heavy slugs, due for need of harmonic tuning.
For you a 28-32 grain .25 should get you a high stabilization factor. SF is often overlooked by BC.
With slugs, high BC means NOTHING if they're not stabilizing at the close range a air gunner might desire to use them at. It's like Gamo saying our gun shoots 1650fps with 5.5 grain Alloy pellets. Whoopty Doo! It won't hit a barn at 100 yards.
Please let me know what your twist rate is, & on what platform you have.
I can tell you that NSA slugs are very good. But I would need to know the length of your slugs. I prefer NSA for PRICE, and shortest heavyweights.
I prefer Griffin 2 stroke extra long slugs for some applications.
Either way I will tell ya I have kept some of my .25's for closer range hard hitting and use my hot .22 slugs for longer range targets.
The Uber Custom special .257 is on its way but I'm done with it all after her. She's going to get tested but destined to be a Safe Queen of extreme rarity.
She ain't no HP Raptor. SHE makes a HP Raptor look like a regular pellet shooter. But again, I don't care about air consumption or that I must be tethered unless I am desiring 3-4 shots.