Hi Greg. I ran limited slips for several years on my autocross cars. First there are several types and your right the 5 speed carrier is a bit weaker than the 4 speed one but anything can break. I have broken tri pod joint outer axle stubs and axles then selves, never a 4 speed carrier or more correctly 1/2 4 speed carrier and 1/2 aftermarket carrier. Upgrading to the later stub axles is a good and strong update, you should already have those with the 5 speed. I had axles made from 302 steel, about as good as it gets and they lasted for 10 years before one of them failed. By the way stock early axles wont get it done, 5 speed axles should hold up pretty well but do have their limits. Now the limited slip. There are gear driven ones, torsen. There are clutch pack type with clutch pack on one axle or packs on both axles. Some also have a cam that tightens the clutch pack and some don't. Some replace part of the carrier itself, these are much stronger, and some don't. A type that was originally was called a phantom grip is nothing more than some heavy springs pushing against two steel blocks that push against the outer spider side gears, not recommended for the 5 speed transaxle. Also some that replace the carrier or at least part of the carrier may also add a second crossbar which is much stronger. Some have a preload and some don't. Yes it will put higher loads on the drive line components and yes you can spin both wheels given enough power. Also the tires are really important since race tires give much greater grip than Walmart specials but that means more load on components. One other thing is the gear ratios, lower gears multiply the torque much greater than higher gears. If you've never figured the torque levels in lower gears verse higher gears you would be surprised how much torque is available at the wheel. Well over 1000 foot pounds. The one that I currently have in my, currently modifying, X19 is a scorpion transaxle and has two axle packs, constant preload, a second crossbar and spiders, aftermarket 1/2 carrier and cams that tighten the clutch pack under heavier load, gas peddle. It works great and you don't even know it's there, it's a ZF unit and it does spin both wheel as much as you want but you come off a corner much much faster. I don't know what type you are talking about or what kind of power, but at least get one with a preload. If done correctly it will completely remove any trailing throttle oversteer allowing you to break right into the apex of the corner and accelerate right off the apex. You will have to make suspension alignment corrections. Hope this helps, there is a lot to take in.
Keep revin
Charlie
) .My experience with torque bias diffs is that they don't really help with such above issues due to the way they work, however, I'm sure there are other benefits and they are definitely easier to life with for road use. 
