I've never tried the HX40, but have tried the others. I had good results from the 35/40 hybrid(Dodgezilla) both as a single and as a secondary with twins. I have been running the S300's as a secondary for a while and like them very much as long as it is 62mm or larger on the compressor. I prefer the 71mm turbine over the 65. I had my worst results with the small turbine, small(57mm) compressor S300. I was running a K31 on the bottom at the time and it would hit 40psi just climbing a slight grade. The 35/40/k31 setup would do about 1/2 that much on the same hill. But had more power at 20psi than the little 300 did at 40. Maybe it was an oddball, I don't know. Bell WSB is what it was called.

 

Didn't the WSB use a single volute wastegate that was on the S200G, if so this would be your problem using it in a twin setup.

 

This is one thing that gives me pause about the Garretts. So far, the theoretical concerns over bearing durability in high thrust conditions have yet to manifest themselves as being real-world problems.

But in a compound turbo application with very high PRs, it makes the Turbonetics design appear to be a lot more appropriate-- the angular contact bearing is very strong on the thrust, and the 360º helps a bunch with damping, as Don mentioned.

In a single turbo application, the thrust element isn't nearly as significant, imo.

jmo