Have you been to tfl.gov.uk journey planner?
I printed out all the alternatives and took them with me just in case we missed the last train. (It was pulling into the station as we were running down the steps, phew!)
We got in quite early - just after 5, and it was a pain that we couldn't get to our seats, so yes, probably would be a good idea to get there later. Most people were in the building by 6:30, so shouldn't be any queues getting there at 7. The arena's quite small too, so it won't take long to get to your seats wherever you're sat.
Also, there were no queues for the toilets after about 7:30.
If you're looking for somewhere to eat beforehand, we went to the Blue Check, which is only about 5 minutes walk away. TBH I think it's about the only place around, but reasonably priced, and they had Kylie on too.