Double XP is a curse on anyone who actually likes the non-max-level-content and who doesn't want that content to become trivial. If you play during double XP you will out-level your missions very quickly. There is zero challenge to missions even when the same level as the mission. I have to be -2 to even notice that I'm being damaged, -3 for any sort of challenge and -4 for a real challenge, so basically if I play a character during double XP the character is ruined.
The joke is also on the designers who went to such pains to balance the difficulty. They talked about it during the recent development blog (altering companions' effectiveness), but did they not know that someone else in the company was making all their work completely pointless by giving us no way to suppress double XP?
I wonder if the person who makes the decision to use double XP to promote the game realises the damage it does. It's lazy (because the effort hasn't been put in to give us an option) and irresponsible (because the consequences haven't been thought through).
Regarding the general XP acceleration, options for that would be good too.
However, TBH I'd prefer to just be able to choose my level cap relative to whatever content I'm playing, whether it's loose mobs on Taris, a heroic or a class mission. I could use -4 for solo and once I'd found a suitable setting for heroics I'd be happy. Then I wouldn't care if I'm level 40 (or whatever) by the end of Nar Shaddaa, because the content would still be challenging. All the content would be open to me – I could play FP's with friends and even hit Social 2 without ruining my class missions.