WiFi solution with ATT's U-Verse 3801HGV 2Wire modem

I'm running out of ideas and could use some help. My speedtest results on ethernet are around 22.5Mbps/2.89Mbps, which are very close to my paid 25/3. My problem is WiFi averages around 5.5/2.89. Up is fine but down is horrible. I have ATT's U-Verse, which comes with the 2Wire 3801HGV modem.

Trial 1: With the 3801HGV - The 3801HGV modem is limited in WiFi tweaks so I've tried everything that comes to mind. I put security on WPA2-PSK (AES), I forced it to 802.11g, forced the channel to 10 (the least used using inSSIDer) and put the power setting to 400mW and put the maximum bandwidth to 54 Mpbs. None of this changed the WiFi performance. It's still averaging around 5/3.

Trial 2: With my WRT54GL - The WRT54GL is a great router for coverage. I installed Tomato for more robust options and easier management. I turned off wireless settings on the 3801HGV and plugged the WRT54GL into it. Wired I'm getting close to full 25/3 but on wireless the results are no different than using the 3801HGV, around 5/3.

Now I'm starting to think it's simply my building. I'm in a 36 story high rise with about 400 units in the building and I'm sure close to everyone is using WiFi. Could all that noise be what's causing all my problems? I really would love some suggestions. I work from home and have webinar sessions throughout the day and having poor wireless performance is starting to drive me crazy.

Any suggestions are greatly appreciated! Thank you.