thanks a lot for your advices on using WiFi modules! I will keep them in mind.
Because of your speed tests, I checked the datasheets of the WiFly RN-171 (http://bit.ly/WiFly-RN-171) and RN-131G/C (http://bit.ly/WiFly-RN-131G).
According to them the RN-131G only supports up to 1 Mbps sustained data rate with UART. With this module it wouldn't be possible to get faster data rates!
However, the RN-171 supports a data rate of 921 Kbps, over UART. And with SPI, up to 2 Mbps.
Most WiFi modules and camera modules with UART only support ~ 1 Mbps at maximum. I think UART can't go any faster! Maybe that's the reason why your WiFi module didn't answer any longer, at such high baud rates?
For our project, the RN-171 WiFi module, connected over fast SPI, would be a nice choice if we really reach the 1-2 Mbps.
At the moment, i have found another WiFi module. WIZnet overs some great serial WiFi modules with "54Mbps data rate and Max.20Mbps Effective Data Transmission". If we only reach a tenth of this 20 Mbps it would be really awesome! :) In their datasheet they also mention "IP cameras" as a possible application example.
Here is the link to the WIZnet610wi module:
Phew, that would be a lot of work to write a protocol stack on your own. The memory wouldn't be the problem, we will use a powerful 72 MHz 32bit controller with lots of flash space.
However, it would be great to outsource this task to the WiFi module, so the WiFly, GainSpan or WIZnet module would be a better choice.