Well, i'm reading different accounts and, not surprisingly, seeing a wide variety of results.
It sounds like the 12x XP bonus does stack with the general experience boosts you can get in the game, but not with other, class specific XP bonuses (legacy, boost packs, etc...).
From what I'm seeing, after the first few planets, folks are pretty much over leveled at that point. Trying to keep gear up is going to be a challenge, but I don't think it's going to matter much. You get some gear rewards from the missions, and, as I said before, you can always go back and farm a few plantary comms to fill in empty slots for you and your favorite companions.
Here is Dulfy's write-up on the experience bonus stuff...
Players who have pre-ordered Shadow of Revan expansion can level up exclusively via class missions (Makeb missions included).
When does the XP bonus start?The 12X XP boost starts immediately when you pre-order Shadow of Revan expansion:
http://www.swtor.com/purchase/item-shpre (you may need to restart the game for this bonus to kick in)
It ends on December 1, 2014, at 11:59PM PST / December 2, 2014, at 7:59AM GMT.
You will only able to see this boost by looking at the class mission reward page on your mission log. Your xp bar won’t be affected.
What boost stack with it?Legacy Class mission boosts and Cartel Market class mission boosts do not stack with the 12X multiplier.
Guild XP bonus (10%) and Cartel Market general XP boost (25%) do stack with the 12X multiplier.
The 12X Boost only applies toClass Missions from 1-50
Makeb Main storyline from 50-55
Can I level up via just Class missions?Yes, a single class mission turn in will net you almost half a level or close to a level depending on your level.
Here is an example: A single class mission turn in at level 47 will give me 224k XP (this is with guild 10% bonus and 25% cartel market XP boost). To level up at level 48 I only need 437k XP.
swtor-class-missions-xp-boost
Will there be a gear gap?Yes, you won’t be able to keep up with your gear with such an accelerated speed of leveling. Make sure to take any equipment upgrades you can get from the class missions rather than the usual planetary commendations.
Luckily Makeb now has a bolster system that will boost your gear while leveling in Makeb so make sure to interact with the GSI looking terminal you can find on every Makeb island.
Costs to upgrade abilities is now goneAbilities will cost 0 credits to acquire/upgrade with pre-order, this includes the speeder pilot abilities as well.
swtor-class-missions-abilities-cost