its really hard to gauge at the moment, since the best playtest i've had so far was probably an 8v8, that was pretty evenly balanced in class distribution which was pleasing, however the main purpose of the tunnels for me were for 3 classes
the heavy to avoid having to cross the middle and be a sitting duck/the spy to get behind the enemy without having to burn all of their cloak, and for the offensive style engineers for setting up teleporters/turrets in more offensive postions, unfortunately i think people need to learn the map a bit more before they fully utlize everything you put in there for them to use, i'm gonna test out the changes/improvements i make to the spawn areas to see how that impacts the general flow.
people have suggested it would be nice to be able to swim from the middle main tower (inside room) to these connecting rooms, but honestly i dont think that adds much to the map.
I think ill run a second playtest version with improved spawns, im hoping this will make the middle area much more chaotic and intense, and then be brave and release a beta and see how it pans out.
hopefully on my days off i should be able to make some good progress on the changes.
edit ,
http://img168.imageshack.us/my.php?image=geooverlayij8.jpg
thought i best attach my uber ms paint skills version, the blue arrow marks the existing "quick" way from the spawn room (its actually underneath the room u can see on the map), the red areas are the portions of the spawn room /corridors i will be adding, and the yellow routes are the hopefully new way to spice up the action, less travel time more carnage

and a couple more options.