I had a very good experience with Jens.  I had a 5150III 50 watt modded to stealth specs, split channel 1 and 2 with concentric knobs, fender clean on channel 1 and the supersonic mod.  Jens was great to deal with. He was very quick to respond to my e-mails and answered all my questions.   I didn't have any issues.  This was back in November of 2020.  It took him about a month to do the work.  Heck he even responded to a question I had about the amp 3 years later!
With regards to the amp, I think the supersonic mod may make it too bright, but the other mods sound great.  The only issue I'm having now (this 5 years later), is one of the concentrics is kind of cutting out, but I can't say that is his fault.