So I sent this to turn 10 Studios but I thought I might put this here as well hoping someone else might have this issue or someone of importance would see this. Any help would be great. Thanks!!
Not really a “bug”. You broke the tune by tinkering around - the default class of the Golf R is what it is and that’s what you reset it to. So, go find a shared setup that you like and install it.
Now, if you cherish the paint job, don’t go removing that too.
What may work is downloading a copy of the original tune, delete it and try again. I can upload the original tune in about 10 hours but only temp as its not really my tune.
But what I did was kept the original tune settings and detuned the build to A class anyway.
Yeah, he messed up the tune, but what he’s saying is that the game now won’t let him install the supercharger that comes with one of the engine swaps. It let’s you do that with the Golf normally, and should be letting him do it now, regardless of the previous tune. That’s the glitch.
Has for a solution, I read something similar some months back. You may or may not find a solution in here: http://forums.forza.net/turn10_postsm30617_I-cant-install-a-twin-turbo-on-my-yellowbird.aspx#post_30617
-edit. Darnit. Satnite ninja’d me 
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OP I have uploaded the tune. Its called Foust base.
Download it and apply to the car. Then remove the tune and see if you can then use the supercharger.
Please post here if you grab the tune or decide not to so I can unshare it as its not my work to share.