greys would be fine providing your engine is in good enough condition to cope with the extra power you'll have once the boost is turned up.

the box and diff should be fine really, diff won't last forever if you launch lots though,but boxes are pretty strong!!!

there are no major disadvantages tbh,although it'd be best to have the car live mapped to suit the greys, MSD now do off the shelf chips for greys conversions,but a live map would be even better