Some of the converted instruments/timbres I did not like and it seemed to totally fail at rendering sliding/tremolo pitches correctly.
I can see that happening. Pitch sliding and tremolo are more advanced things that may be trickier to properly convert.
For the instrument conversions though, it looks like OpenMPT is actually directly reading from the same soundbank file that the Windows MIDI driver is using (namely c:\windows\system32\drivers\gm.dls), so in theory it should be the same underlying sound samples. I did notice though that OpenMPT doesn't seem to apply the correct amount of decay to sounds like piano and guitar that should decay over time, so long notes from those instruments will tend to sound subtly different. Perhaps other instruments used in Paintball have worse conversion issues.
(It's also worth noting that the MIDI format has more freedom than tracker formats in terms of how many simultaneous notes you can have on the same channel. I can imagine the more simultaneous notes "jammed" into the same channel in the MIDI, the more problematic it might become to convert. So perhaps the Paintball MIDIs also happened to be put together in such a less conversion-friendly way.)
The OpenMPT wiki on the web did warn that MIDI import will not be fully automatic and will likely require some post-conversion fix-up work. If nothing else, I think you'll need to setup the looping yourself since I don't think that data is captured in the MIDI format. I expect that we may also need to look into adjusting the MIDIs to perhaps avoid or minimize use of certain features or instruments that just don't quite convert as well as other ones when using OpenMPT.
Also note that when saving the conversion in OpenMPT, you should only use the IT or XM formats (based on further reading of the manual, IT may be the better choice of the two), or OpenMPT's own format (though we might want to avoid that one since I'm not sure how easy it is to find code supporting that format that Simon can use for Lix). Other formats are older and more restricted in capabilities, and will likely introduce further issues into the conversion.
If I have time maybe I can try to convert the rest, and post the results here to let others have a listen. I will try to at least make the tunes loop properly, but won't try to fix anything else post-conversion.