unless you ring up and ask, the manufacturers wont release technical data (the rate of dampening). Therefore you have probably only guesswork to go by, unless you know someone in the trade.
All decent aftermarket kits should have stiffer dampers than standard, though. The difference between FIAT and FK shocks was unbelievable when i compared them. Im sure others will testify that whatever shocks they have are much stiffer than standard as well.
Lastly, you dont "need" the strongest available, you just WANT the strongest available
if all its for is to help prevent rubbing (which still might not be cured, even with the stiffest shocks on the market) i would just buy a set of what picks your fancy.
i would be plumping for a set of these:
http://www.carpartsprovider.com/Fia...34561-KO8040-1272Sport-Koni-Sport-Shocks.html
which i think are adjustable anyway, but even if not, they are some of the best shocks around anyway
one other thing - you will want front shocks as well, to balance out things. A harder rear end = tend to oversteer, a harder front end = tend to understeer. A set of rear-only hard shocks may turn it into a deathtrap when cornering, dunno. I dont think many people have tried it before.