Solid mounts, especially when cammed affect other things...like brackets and components in the engine bay.
Ask me how I know with a 625-650 idle on a Thrasher Cam with poly mount everything. Header bolts shake loose, even Loctite'd...snapped a bolt on the AC Compressor bracket...timing chain broke. It isn't fun. Poly's are rough but I couldn't imagine running solid metal in there. There are a reason people run poly uppers and solid rubber lowers...there is a little "give" to the drivetrain parts...same with a flex pipe in your exhaust system.
To add, an XP isn't near big enough of a cam to shake crap loose, so...I guess carry on.
