boost leak??

My car stumbles at high boost, I don't think it's fuel cut because I've
been searching this forum and reading other posts about fuel cut and
my car doesn't suddenly jerk to a halt or anything like that, it just stumbles and backfires for a split second then continues accelerating. I kept my eye on my boost gauge and noticed that this stumbling only occurs when I reach 20 psi. Alot of people on this forum suggested that it might be fouled plugs or a boost leak. I ran my car at 16 psi and it ran perfectly fine with no stumbling or backfiring. Shouldn't my car stumble all the time if I had a boost leak or fouled plugs?? Also I should mention that I recently just installed 550 injectors and safc. I have my high throttle corrections set to -15@1k and gradually decreasing to
-18@7k. I'm planning on installing an aem wideband o2 on Monday so I can monitor my afr's. Anyways, I just wanted to know what you guys think might be the problem??

93 tsi awd
-rewired190
-uicp
-k/n filter
-mbc
-2.5 turbo back
-550 injectors
-safc

get NGK BRES's and gap them at .028. Get new wires. Don't boost that high on your turbo without more mods.