So I have a 23 Sith Marauder and a 17 Jedi Sage and Jedi Shadow. Which of the 3 will put out the best dps at 55? Does this game allow duel specs? Also is there anyway to change race on them?

I believe the marauder would kick out the most DPS at end game.

No dual spec.

Yes it's an in game feature located on the fleet at the Cartel Bazaar ( I think ). Just ask in general chat on the fleet.

For PvE a good indicator what's possible are some server leaderboards, for example: http://www.swtor.com/community/showthread.php?t=685275 (not my server and little bit outdated)
Marauder, Sniper and Commando are always top/good DPS since release, plus Marauder is the only class who can provide a DPS buff "Bloodthirst - all damage and healing increased for 15% for 15 seconds"
Sage/Sorc is way better since patch 2.0, but still little behind compared to some others.
Assassin/Shadow can put out quite some burst, but is lacking consistant DPS for longer fights, but will recieve a buff with 2.5 (which is quite nasty for PvP, cause Shadow/Assa is already very good in PvP)

nCrank is right, there are no dual speccs, but you can buy a mobile respecc for 200k (or some cartel coins) in the legacy menu, so ur able to respecc anywhere (outside of battlegrounds since last patch)

Race change is possible at the Cartel Bazaar, but u're not able to switch factions.

Good luck!

You mean pvp or pve?
Pve I guess the sniper and merc does pretty much the same dps. THe marauder is maybe a little more behind but because of his auras, he is as usefull. Remember pve is mostly single target damage.
Pvp, well it depends. In the new 4v4 warzones, the marauders and jug dps are clearly the best.
and if you want to know what the dps looks like for pvp, a very good game as a lowbie ( below level 55) is something like 700k damage per second. A very good game for 55 is 1000.