I have MMP inlets and outlets and like them. Seem fine for what they are.
MMP also got the 'other vendors' to stop taking insane margins and ripping off customers. Inlets used to cost $500+ and now sell for $200 with a 'fair' profit margin. For that reason he got my business.
You can also buy a lot of N54 parts on Alibaba:
https://www.alibaba.com/showroom/n54-silicone-inlets.html
https://www.alibaba.com/trade/search?fsb=y&IndexArea=product_en&CatId=&SearchText=n54+downpipes
the plumbing looks good, did you make some changes?