Well for adsl2+ you really want a broadcom chip, this plus that fact that the gt is more powerful than the standard dgs is why it's so popular. The fact it holds connection where other routers can't/won't makes it the preferred router. For adsl2+ isps will tell you to avoid the v5 and get a gt, that belkin 7633 however should be fine on adsl2+.
I'm on be with 39/21.1db with an snr of 3db and get 13.1 sync, on a 6db profile i get 11.5 (10.5 throughput) and 9 i get 10. That belkin should be achieving similar, the v5 simply wont

It looks like you're on 9bd but without more detailed line stats i.e. error count (fec, crc etc.) which i don't know if you can get from the belkin, you can't rule out a poor line.
In any case, congested servers coupled with small files don't give speedtest.net the most accurate results, use the
think broadband test files to get a better view of your throughput (larger the better)