Hitches don't need to have extensions to fail. There have been many documented cases of hitches failing for other reasons.
In any case, you have to keep in mind that the max rating is for the extension. Reese, for instance, rates their extensions. Take a look at their site, but here are some examples. NOTE: they are way under SuperHitch ratings for the same lengths.
http://www.reeseprod.com/products/hitch-accessories/other-accessories/trailer-hitch-extension/hxyNMzUEThezO4ZpzhR4RkmmwA%7CEZYPLTitanĀ®, Receiver Extension, 2-1/2" to 2" Extension
24" Length 600/6,000 lbs. WC & 800/8,000 lbs. WD
34" Length 450/4,500 lbs. WC & 600/6,000 lbs. WD
41" Length 500/5,000 lbs. WC & 750/7,500 lbs. WD
48" Length 400/4,000 lbs. WC & 600/6,000 lbs. WD
Some SuperHitch numbers with Truss for comparison.
SuperTruss Extension
48" Extension And WD, 1,200 Lbs Tongue, 12,000 Lbs Pull
48" Extension Without Weight Distribution 600 Lbs Tongue, 6,000 Lbs Pull