3.5G is another way of saying HSDPA so the symbol is equivalent. As for the actual speed it depends on the actual implementation and if the company reports values with overhead or without.(the N95 also was 3.5G but it supported only 3.6 mbps and IIRC the N97 went up to 7.2)
I don't know why overhead is so much but it is on wifi too. The real throughput of 892.11g is just over 25mbps, and you can test it by moving a large file over wifi
I cannot now find where was that article that explained all this but I remember that the HSDPA class of the N900 was listed as 21.1/10.2 so it may very well be as fast as your SGS2. Nokia has always been quicker than the competition adopting new 3G speeds.