I haven't benchmarked the game comparing GCC versions but I haven't seen any gross slowdown. You can still upgrade to 2.05 and keep the older GCC I believe.
In any case, at one point the ogg vorbis libs were still the GCC 2.9.5 versions and it worked fine in q2dos so compiling your custom mods may work okay from an older version. You could test it by posting a dxe of the core gamex86.dxe and I'll test it out.
I would only be interested in something like a mod named roguebots or something to this effect, but I'm not interested in injecting bots into all the game dlls. Something that might be worth doing at a later date would be expanding the game DXE itself it to load a separate DXE like bots.dxe and add bot ability as a drop in and keep the code bases separate. I don't know how hard it would be to do however. The import/exports might get out of hand doing it this way.
What are the concerns? Most of this code is from yamagi quake 2 which has for the most part been tested pretty well and has a lot of sanity checks that prevent complete meltdown of the game and in DOS that could be a real concern.