It is probably true that non-floating axles experience more heavy side loading and wear. However, any number of us on here have mercilessly abused the 8.75 rear over the decades without detriment. Can they break, sure, but in many cases they are more than adequate for the application, especially when they are not being shock loaded like in a drag racing environment.

I have used them in oval track and drag racing since the '80s. I've never broken one, but I've also never dealt with more than 400-425 horse mills, and even with that I used a spool and 35 spline axles in my drag application. Since this was using slicks on a prepared track, I thought it a wise investment. On oval tracks, I used standard axles and sure grips. This has been both with treaded and slick tires. However, oval tracks never approach the level of grip you find on the starting line of a drag strip.

Will the 8.75 stand up to autocross, IMO, absolutely. Are there better components out there than stock, also affirmative. I've got no experience with the Eaton Truetrac but it looks like a good piece. To me, an axle swap at a later date is not a big deal that would require a lot of effort. If you want to upgrade now in anticipation of future power levels, you certainly could or you can wait until later. It all depends on your budget, effort you're willing to put in, power levels, and application.