screek said he already did some mapping for q3, and he will tell me the things he knows...
if you decompile a map, you need new textures, new lighting and maybe also new vis-blockers...
vis stands for visibility, and with vis-blocking stuff you make sure that urbanterror just renders what you really can see instead of the whole map, else you most certainly get framedrops... example:
http://www.forums.urbanterror.net/index.php/topic,7427.0.html and theoretical solution:
http://www.modwiki.net/wiki/Visportalin other words: simply decompile, new textures, new lighting most certainly is not enough.
vis-blocking issues may be the reason ut4_suburbs has such bad fps? another problem may be that weapons and models that you cannot see are rendered, too. i don't know enough about this subject yet, i just mention it so you get the idea that brushes, lightning and textures are not everything you have to check.
so... i am back to searching information...
edit:
For 1.6-maps there are two I know of:
WinBSPC
Bsp2map
Both of these programs will decompile a bsp into the .map-format, which can be opened by Hammer.
Warning: The map-file created by the decompilers is crap, you wont be able to re-compile without re-building the whole map. Decompilers should only be used to learn how a map was created not to steal other peoples work. If you want to edit a map you should ask the original author for persmission, and if granted, ask him/her to send the original rmf-file.
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^^
seems like i slowly get to the bottom line of it all.