Yes, the part purchasing screens are essentially unchanged since
GT1. Well, except that they added that phoney-baloney "processing" if you fit them. (So don't fit parts when you buy them; fit them in the settings menu where they ping on instantly--that's why I said "phoney-baloney"). So the parts screens could do with some updating. Really, what does it mean to "unfit NA tune"? I see even they realize three options of part are silly for many cars. But still what's with this 100 bucks price difference in parts here-or-there? If they want to affect your strategy, then perhaps 1) certain parts should usually not be available for cheap cars or 2) the price difference should be greater between cheap and expensive upgrades. I.e. Cr10,000 for brake controller for Miata, but Cr30,000 for a Viper or NSX. Perhaps it doesn't make a lot of sense based on real life (but it actually might) but I think it would affect gameplay more the way the want to.
Okay, so how to speed up the purchasing process?
1) All parts are available on one scrolling screen (inapplicable or already purchased items greyed-out)
2) You check/uncheck a checkbox beside the ones you want
3) A running total is kept, which goes red if you would be overspending
4) Whenever the total is not red, you can agree to purchase
"Permanent modifications" would be done at that time. It's not clear you need the ability to fit other parts at purchase time, since it is quick and easy to do so in the settings screen. That avoids the issue of which Turbo Spec to fit if you bought more than one, etc.
But also regression of the settings screens back to separate parts change and adjust screens would be nice. It shouldn't be so easy to throw away your settings by accidentally re-fitting a part!!!! And why did they do away with the
GT3 settings files? I had a whole collection of them for my F1 cars and it certainly beat this SetA/SetB/SetC nonsense. If nothing else, you could make a "backup" of your settings in case you accidently set them back to default!