I'd go with the offhand personally, though I'm a bit biased as I'm a sword rogue. But as to how much damage you lose if you're specced (and use) maces as opposed to swords here's the breakdown: If you use the fool's bane and merciless bone cracker. Then over the course of 150 seconds (2.5 minutes the length of a farmable boss.) Then the total damage of these two weapons (without accounting for attack power) is 27,775.
If you use the merciless offhand and vindicator's brand it will do only 26,823.1 but after we account for sword specialization they do 28486.7
If you use the merciless offhand and merciless slicer it will do only 29303.8 but after sword specialization it will do 31306.5
This only an estimate and it's nowhere near close to actual values but if we take other factors from actual combat all of those factors buff the sword bonus damage and none of these factors buff maces. For instance I have only accounted for white non-crit damage, but if we factor that in it affects both swords and maces and becomes a wash. But if I were to have included attack power then the white hits still become a wash with the exception of extra swings, meaning that the extra swings will actually do more damage than I've accounted for here. Roughly 2002 additional damage (calculated with 1550 attack power). The most compelling factor, of course, is that Raid Boss fights are usually longer than 150 seconds. Nightbane for example tends to be a longer fight, and so there will be more extra swing potentials for swords.
In conclusion, if you go maces you will still be able to lead the charge in DPS you just won't be on the very top, and you certainly won't beat a sword rogue. If you do decide to upgrade I would honestly go with the off hand weapon first. A new main hand is nice, but it only gives you 16 dps more and 43 more higher damage than your current mainhand. Additionally, there is no offhand weapon that's going to drop for you in Karazhan and the only time, if I'm not mistaken, that I think you can get an off hand that's better than what you have (save for the arena OH sword) is in SSC, which may be quite a ways off.
I hope that helps and if you have any questions about the theory-craft/crap let me know.