Tx power should have almost nothing to do with throughput. It will however affect distance you can be from the access point. As long as the AP can clearly hear your tablet then the speed going to be the same whether at 100mW or 10mW.

The a change would only affect re-transmits if you are at a fringe/marginal distance from the router/access point. In such a case switching to the higher Tx power will actually increase battery life as the card is not working as hard to send the data repeatedly.

Overall I have found the speeds fine for the card. Unless you are concerened with LAN speeds vs WAN speeds. Unless you have a super wide broadband pipe whatever the LAN speed is will be faster then your WAN connection can provide data. Again, unless there is a distance issue and a lot of re-Tx's on the LAN side of the equation. To fix that there are two options I can see. Either increase the Tx to the 100mW setting or if that fails move closer/relocate your Wifi router or it's antenna orientation/location. I guess there are a couple added options but those would involve adding more LAN/WLAN hardware.

Oh, also just thought that you should make sure the issue is not in the router and check the Tx power setting on the router itself. Some routers do have the ability to adjust their Tx power too.

EDIT: I re-read your initial post and wanted to and an FYI. The to most significant factors in the actually d/l speed to your tablet will be the CPU & NIC...I do not know if the NIC in the table has it's own processor or hands off processing to the system CPU. Either way neither will be in the same class processing wise as your other full sized systems. I get significantly slower speeds on my tablet then my other 'full size" systems. But that is to be expected and completely a hardware issue. You might be able to tweak a setting here or there to squeeze a few more bps but it really is unlikely to be that significant.