Very hard Q to answer I think some of the tunes would be going hand in hand... say you only do a plenum spacer and gain 9hp if you then add pop charger and gain 4hp on top of that you might see that if you take of the plenum spacer you'd only gain 3hp etc. Not sure that was a good explenation but there you go
My own experience in the area have seen a lot of effort and time for very little hp/torq increase my car was using haltech ecu and tuned with e5 (higher octane) so it was running like MENTAL, when I was able to get proper fuel that is... most of the time it was knocking and throwing up engine lights. I've now got a retune with RON95 and the car is producing a very low margin higher then before (torq higher though) but the car FEELS very different more driveable and more pull in the gears.
After the E5 tune I added on two race cats, custom exhaust system and kinetix ssv plenum not sure what they would give individually according to their traders but the custom exhaust deffo made the engine breath easier and so did the kinetix plenum and the race cats. < I think for an NA tune they all might be overkill but since I'm going FI (on the roadmap anyway) It will all play an important part later.
(SSV according to their site gives 30+hp (!) and the cats might give another 10 and possible 5-10 on the rest of the exhaust)
Btw, not saying that any of this is the law... more a what I feel / think in the matter. Please feel free to correct me and add your own input.
REALLY want to go down the E85 route....
A.