T2 won't be very different from T3 as Immigration waits are unpredictable everywhere. By my calculations, and assuming your flight is on time, you should have plenty of time to make the connection. I would plan on taking the Tube to St.P - it's cheaper, it's direct and it's a shorter walk at LHR. However, if you are running late, then do HEX to Paddington and change onto the Tube (or taxi) there - it's massively more expensive but should save 15 minutes off the travel time, albeit adding significantly to the hassle factor.
If the plane is late and you are running a little tight on time, HEX plus taxi is probably about the same time as taxi the whole way, so I don't see the latter as being a viable back up. Of course, the other possibility is a cheap back up flight to Paris the following morning - you can usually find fares so low that they are essentially throwaways.