Like is said above, I have no experience with the proprietary speed boosters, so this is generic advice. Maybe someone who has actually used the equipment you are using will eventually join in here.
This is help diagnosing the problem (not solving it), but hopefully the results will begin to point to what is going on.
First, turn off everything except your router (keep the internet connection live) and one computer with wireless. Turn off all wired computers and any other wireless computers. With just that simple "network", test your speeds with standard 802.11g and then with your afterburner features enabled. If even this simple system slows down, then you have something seriously amiss. I "think" (caveat - my inexperience with the boosted wireless) these systems will slow down if they detect standard or 11b clients. Are there any of those within range of your network?
If this simple system is slow, then try turning off the computer you just tested and turn on a different client. How does that perform?
If this simple system does run at boosted speed, they try turning on another wireless client. If the system slows to a crawl then, then there are a couple of possibilities... maybe the wireless card in the machine you just turned on is defective or not in the high speed mode... maybe the router is becoming overloaded (cheap processor?) and can't keep up with more than one high speed client.
Basic guideline: make as simple a network as you can and see if that works. If not, then the number of things that could be wrong is simplified. If it does work, then continue to make the network more complete until it finally falls over.