If you go GT40, yes, you need top and bottom....your stock lower is only good for the stock upper...The GT40 has different port...
Send Tmoss a PM..he usually has stock lower all ready to go...just send your back...
I felt a big difference in the swap, low end, but up top mostly...pulled all the way to redline no problem....as mentioned, whats an a/noon to swap it out...I did the LTs/Prochmbr, FMS65, u/drives and Toms lower...195RW stock to 233RW afterwards was noticable to me...
Like I say, its an option...with what your already running for mods, youll be surprised what that ported lower adds...As usual it a money thing...A TF is nothing to sneeze at and a nice pc to boot...
I recently swapped one of Toms lowers for this 93 Cobra he did for me as I did the Fox swap to....I gained 12 hp from the his stock ported lower to his Cobra...That says something to me about the stock ported lower...it does work...