Yeah, you probably got it figured. Once the weather cools down you'll know for sure. The heat messes with everything. I took my guns out today and they instantly started sweating, man - o - man does that make me cringe. The hot sun was one of the reasons that I finally decided to paint a scope. It has to be better than leaving it black.
The plenum is certainly going to help balance out your power tune. I will be interested to hear the numbers for that. When I was playing around with different plenum sizes with my Paradigm, and actually looking at the real numbers and doing the math, the only size that I could use the extra volume effectively with was the smallest. Not what I wanted, but it was what it was. I wasn't porting that or changing springs, it was too close to perfect to do crazy stuff to.
I shot my 250z about 300psi below my reg setting today and was still hitting on target at 95y on my 2" reaction targets, I thought that was pretty good. I am just really happy with how this gun shoots. I am done with it too. I just got a case for it and I am going to put it to work next week shooting some woodchucks.
This was the first airgun that I have worked on that has changed my opinion of my other airguns, by a lot. It's performance with slugs, coupled with how well slugs work in my .177 Vectis and my Paradigm, has me starting to hate shooting pellets. I am just losing patience with them and don't like putting up with all the extra drop and drift and bleeding off of speed. Mostly with just the .177 and .22, I still like the 34gr .25, and the big .30's are just entertaining ballista. I shot my FWB124D's today for the first in a year, I think. I just don't like shooting springers any more. The banging and the noise, compared to a near silent dead shot cycle pcp, no thanks, I'm done. I find it amazing that with all they have going on they still manage to be accurate but I don't enjoy the experience like I once did. I don't like loud pipes or music any more either. The thing that bothers me the most though is that there is just enough vibration that I can't see my pellet flying most of the time. I only shoot the springers off hand, so that has a lot to do with it too. Anyways, it is a little bit sad to end a good project, but it's great to end up with an awesome shooter. I am not planning to do anything for a while now, besides finding a slug that my TalonP likes, and that 's about it. Good luck with your plenum!