Boost problem.. need some advice!!

[NUSPEED_WOLF]

So I recently got a used 2015 FiST an I kept reading as much as I could in these type of forums an I'm having a boost issue.. I feel as though I lose alot of top end boost at 4k, yesterday I took the front end off to find an inch of what looked like tar on both rads.. I cleaned them off, even tightened the hoses an still feel like I'm losing power somewhere.. anyone have this issue or know what I should do?

 

[PlatinumFiST]

Do you have any pictures of what it looked like before cleaning it off?

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[NUSPEED_WOLF]

Yes sir I do, forgive the mess it was done at my brother's place