Have flown a C210 with IO520 back to back with a 550 and there is sod-all difference, apart from a little more 'willingness' at higher altitudes; noticable particularly on taking off from high DALT locales (TWB in this case).
One had a very nice panel.....
the other had factory aircon and a leather interior.
BUT Chucks 'new' Bonza with the EFD1000 panel is gunna be a real winner IMHO....new IO550, new interior, new panel, new paint....mmmmmmm!!!!