Main downside is that you will start a new 18 month contract.
I think you won't need to pay more.
The p2p throttling will be similar; many people are actually seeing fairly fast p2p speeds, much better than the throttling t&cs state.
Your download may be quite a bit faster (type your telephone number into http://www.productsandservices.bt.com/consumerProduc...
to see an ESTIMATE).
You upload will almost certainly be a lot faster.
Infinity reliability is pretty good. People do see speed drops on Infinity. Unusual for it to be so bad it is worse than ADSL (for example, mine is down from 58 to 37, which is still double what I had on ADSL). Apart from one bug
which is hitting some people who need to reset the router fairly often to get download speeds above 1Mb.
Some people see faster pings on Infinity, many see much slower pings (eg my BBC pings went from 19 on BE to 35 on Infinity).
If I were you I would upgrade. I was on a better ADSL package than Option 3 from a better ISP (BE) for less money than Option 3, and I decided to upgrade and I am pleased.
Moved (with trepidation) to BT Infinity 2 for upload speed. Happy BE user for several years.