You’re in a tight spot.
The ESI should be avoided because it’s class compliant and doesn’t have dedicated drivers. The drivers in Windows ain’t that great. Dedicated drivers are needed for better performance - especially if you’re using 8 synths at once. The generic windows midi driver has 3 problems: no multi client capability, corrupted SysEx transfers when large blocks are transferred, 1 mS latency between each event when they are sent over multiple ports. Using an 8 port box with many synths hooked up - that’s a totally unacceptable amount of latency.
Motu - I hear mixed results (some negative like you mentioned).
Another contender is the Roland UM-3G. This is a 3x3 unit, but 3 units can be chained together. I’ve had a hard time getting reports on these - limited info was that it worked well. However, this unit will not have multiclient functionality when Cubase is using it (had Sweetwater test this out for me).
I’m in the same boat as you. Hanging on th XP because I don’t know where to go with midi. Really sad state of affairs. I would pay a good chunk of money for something solid to replace my two midex8s.