Main reason for suggesting a Google Mail account is that it's free, and has plenty of capacity (so if some friend sends you 50 photos from a holiday, it will cope easily). But more to the point, if you ever decided to switch ISP, you would not be in a position to have to then start telling people about your new e-mail address. It's enough to stop people leaving.
In my case, after having a mail address for 8 years, the ISP was taken over (bought) and I lost it in weeks without any control over the matter. So I went to a commercial e-mail service (Runbox.com) and have tried others too (but GMail is pretty good at filing spam away in a folder, has oodles of space, unlike one other account I paid for which has about 50 MB total, and is FREE).
Regarding speed ... They could switch you from "up to 8 Mbps" to "up to 16 Mbps" or "up to 24 Mbps", I guess. There can be improvements if they switch from older BT kit to newer LLU kit, but there are few guarantees.
If you have run any speed tests, or know your router connection stats, it is possible to give a better idea about whether the speed would be higher afterwards, but if it is fast enough at present, it's really a way to lock you in for a while, so they cannot lose you to BT or some other ISP.
If you go to www.SamKnows.com and find your exchange (or give the first 7 digits of your phone number, including the '0') then it may be more easy to determine what AOL can offer for people on your exchange. Whether you see an improvement depends on distance from the exchange - anything under 1.5 km can see more of an improvement than further away.
A look at the graph online here may show just how much/little the speed can increase... Further away, even if the maximum speed (with ADSL 2+) is 24 Mbps, the user sees minimal improvement.
Don't worry if this all seems like hot air, niftyfifty, but the point is there's not really a big guarantee of seeing higher speeds (unless your speed is at the upper end of the range already!)