After collecting a few ideas that I've found here on the forums and adding them together, I've came up with this idea for you all to use. It's just a spreadsheet, but it's much more than that. It's a simple tool, but also very useful. This was made using Google Sheets, so best results would be by using it in Google Sheets. I'm not sure how well it will work in other programs, although they're all about the same.
Fill in the WEIGHT and HP fields of the part you are upgrading and it will tell you which part will benefit the car most(Stock - Pro 3). This is a specific part by part(Displacement, Turbo Up Pipe etc.) calculator, most useful for maxing cars. All you have to do is give it the raw information and it will do the calculations for you.
When using this it's best to have your car at least Stage 2 everything or some parts may change the outcome and consequently cause you to have incorrect parts later on.
Link to use it: HERE
I'm not sure whether it will let you guys directly edit or not, so you may have to create your own spreadsheet and copy paste this into it for it to work.
If you have any questions feel free to ask here or in a PM. Good luck and happy tuning!
Here's a preview(it looks better in Google Sheets):
PART TO USE