Consider that
1. Wireless is a half duplex medium
2. Wireless is a shared medium
3. Wireless is prone to disturbances
802.11g has theoretical maximum throughput of 54Mb/s. However because of the limitations posed by the abovementioned, in real-live environment reaching steady throughput of 30Mb/s is good performance. Since it's shared, in principle 2 g-clients get 15Mb/s each, 10 get 3Mb/s each etc. 128 clients is the radio maximum, but you can't really put more than 20-25 / radio and expect any kind of usability.
The problems you're experiencing with 10-12 clients migh be due to interference, or old FW version... or something else. Remember that MSM310 is not any kind of performance powerhouse. I personally wouldn't even try more than 10 with those.
There's really no way to find out anymore when first of these were produced. Colubris was already selling them before HP acquired it, so...