I have now done some testing of USBASPI.EXE v. 2.28, USBASPI.SYS v. 2.27x, and USBEXFAT.COM. I found no reason to prefer 2.28 over other versions. However, 2.27x loads much faster than any version I have tested previously, so I definitely prefer it over other versions. USBEXFAT.COM works about equally as well as DI1000DD.SYS, except for file name searches. A search that takes 35 seconds with DI1000DD takes over 5 minutes with USBEXFAT.COM. For large file copying, USBEXFAT may be a little faster than DI1000DD. USBEXFAT gives some flaky behavior. If it is loaded before Smartdrv, searches of the USB drive are much faster, but searches of the hard disk are slower and noisier.
My Win98se system is 22 years old, so the hardware technology is not up to date. The USB port supports only UHCI, and is substantially slower than USB 2.0. USB 2.0 supposedly can transfer 480 Mbps. On my Win98 USB, I see rates of 1.6 M Bytes/sec. I'm not sure of the definition of Mbps. If it means M bits/sec, rather than M Bytes/sec, then 480 Mbps is really 60 M bytes/sec.
I have also tested ASPIDISK.SYS, version 4.01 by Adaptec. It works about equally well as DI1000DD.SYS. It is dated in 1997, so it may not be as up-to-date on FAT32 as DI1000DD.SYS.
Incidentally, I have tried loading the drivers after bootup by using DEVLOAD.COM. It appears to work, but something goes wrong shortly after that causes a reboot. I can use internal DOS commands without causing a reboot, but running external programs causes out-of-memory errors. So, it is much better to load the drivers from config.sys. This does not apply to USBEXFAT.COM, possibly because it is never loaded by config.sys