The saloon uses different hinges with springs attached, its not just the gas struts that are different.

I cant imagine its down to high pressure struts or you'd never get the boot closed! I've seen a few where people have fitted an engine valve spring around the shaft of the strut that gives it the upwards momentum when the boot is popped. IIRC it was only a factory option for auto boot on the saloon, so its a 'bodge' of sorts.