That's also what he told me in a phone call prior to our 8329SS purchase. Yes, the cheap tires they put on on their trailers are rated for the RV's maximum weight, but the tires lose capacity as they age. Better to have excess capacity to account for that fact.
ST225/75R15 LR-C tires have 2150lbs of capacity each for a total of 8600lbs.
The Rockwood website says my 8329 has a GVWR of 8850lbs, minus 883lbs of hitch weight means those tires will have 7967lbs riding on them if you load to the max GVWR.
That gives each tire 158lbs of excess capacity or 316lbs per axle, and that's if you manage to load your gear so that each axle has equal weight. Keep in mind that this is when those tires are new.
Upgrade to LR-D and each tire has 2540lbs of capacity, excess capacity jumps to 548lbs per tire and 1096lbs per axle which is almost 2200lbs of total excess capacity. Also if you buy a name brand, you'll likely have a better tire.
Rockwood is correct that upgrading to LR-D tires won't increase the capacity of the trailer or the axles. It will decrease the odds of a blowout as your tires age though, and perhaps even before.
2017 Open Range - Light 297RLS, BX1 Gooseneck
1997 Dodge Ram 2500 CTD LB 4x4