Not sure about the Topun 2's, but since the drivers were updated a couple of months ago the original works much better, so based on the originals:
1. They work great in MAME, assuming you have all the correct options/flags set in MAME itself. It took me a while to get them working but with the new Topgun utility it was easy. You will probably have to remap the mouse axis in MAME using the guns as they usually identify as 'mouse 4'.
2. The sensor bars on the original are about 14"/15" long, but only about 3/8" wide and less than an inch deep.
3. You can mount them behind a clear bezel at the very edges. Precise location is flexible as you can calibrate the guns based on where you mount them, so long as they're not over 40" apart. However, they are greatly affected by reflections/interference so even though you could mount them behind smoked perspex, you may lose performance.
4. The wires from the sensors are thin but are plenty long enough to stretch to the PC. Mount your bars correctly and they are easily concealed.
5. The gun wires are fairly long too; I'd say you could stand around 6 feet from the machine with no trouble.