Is not the major $$ and downtime of the panel conversion not enough 'horsepower'?
From a human factors standpoint I think the ProLine21 is better - much fewer soft buttons and nested menus to page through. I can't see what operational advantage there would be w/Garmin either. Perhaps you get xm weather with Garmin but I'm sure there's an upgrade to the PL21 that can give you that and these days you can probably add in a bluetooth ADS-B box so you'll get real-time wx, traffic etc. bluetoothed right into your iPad.

Tell him to spend the money on your salary or a Full-service contract w/Flight Safety or an additional pilot, or a mechanic or on-board wi-fi or. . . so many better ways to spend that, what, $100k? $200k? panel re-work.