Cos it's already tuned most of the work should already be done. You might just need to get some more fuel happening earlier and pull some timing earlier but eh.
There's enough info out there that providing you dont just try and boost the fark out of it without testing like another member recently did, you should do alright.
Up to you mate. It's your money and your car. You call the shots.
Obviously if your car can't produce a result due to farked coils, original stock fuel pump or similar then fair enough, he's just preventing you from paying twice so to speak.
I call mine that when it breaks down
There was a conversation on here somewhere that was talking about the height of the new bearing mount. Apparently shimming it to make the shaft run straighter (lower to the ground a mm or 3) is a thing.
I haven't measured or tried it myself but it seems possible that a non genuine part could differ slightly from oem.