imran,
wm5 is kinda experimental, very cpu intensive if you ask me.

wm6 has better performance, cosmetics upgrades, more internet connectivity and compatible with most wm5 program.

its quite possible to change wm5 to wm6,

but its all depends on firmware/rom availability, check your devices with xda developer forum

yup, hes right
wm6 could avoid the system hog (the compaction issue) , twice faster response than wm5

it's depends on rom availabity,
some of them still dont have support and not much keen xdadevelopers geeks willing to 'cook' it