I think a stock 31 spline vs a stock 28 spline is like 30% stronger. I still havent found that chart I had that stated axles size vs strenght increases.
Also a lot depends on materials useds most aftermarket axles are 1541 but thereare some others something like 4140 or something like that. the problem is the harder the steel used the more likely it is to just break. Axles tend to want to twist a little when torque is applied stock axles twiest a lot and over time may retain the twist and eventually fail. aftermarket axles general still twist but a lot less so they tend to break quicker of course it usually takes a lot more to break them.
You have it a bit backwards...
You are correct on the 1541, infact I don't think the aftermarket companies even make a weaker material anymore but 1541 is a reasonably strong material.
4340 is the axle material when you step up the strength chart. The next step is 300M.
The more you pay (4340, 300M) the more twist they can absorb before failure. 300M can twist up to 180 degrees...They will always be HT (if from a quality seller) and many will also be Cryo treated.
I haven't seen it mentioned yet but there are also 40 & 47 spline variations available. Most desert trucks use 35 or 40 splines. Many Trophy Trucks use 47 splines.
A big step would be a Full Floater conversion to take the vehicles weight off the axle shaft which only creates a pre-load. Losing that pre-load will give you a less strength needed situation if you follow me...
