Everything you hear, you have to take with a pinch of salt.
Some changes will make some areas better and make some others worse etc etc.
Its the owners choice.
I hear about Toyota spent x amount of research blah blah, but that was along time ago and some areas (not all) has moved on and and improved since then, so some areas/mods will improve the car than the original parts ever did.
It all depends what and how the owner wants the celica to perform.
I would be careful what some of these garages say also, as the mentioned garage at the start of this topic thought they knew all about the 7afe engine and they didn't.
Go with your instincts, weigh up the pros and cons and do what you want............its your car after all not theirs.