12 inch subwoofer overkill for desktop setup?

First of all, why do you think you need to replace your current sub?  

What kind of volumes do you listen at? 

I have a nearfield (not quite desktop) system with Harbeth P3ESR about 4 feet from my listening position and a 15" 725 watt subwoofer on the floor right next to my desk.  My room is about the same size as yours.  I listen at moderate volumes and you'd probably never know the sub is there unless I was listening to some EDM. 

It doesn't "overpower" the room.  These things come with volume/level controls that allow you to integrate them with your speakers.  I just have a lot of headroom and that sub will never be strained in any way.  It's barely "idling" if you were to compare it to driving a car.  It's definitely overkill and I could get away from something smaller.  I previously had a less powerful 12" sub in this room and that also worked well.

I'm a proponent of multiple subs (I have 4 in my main system), but sitting that close to the speakers and sub, there's no need for an additional sub.  Having multiple subs in that small space will have little, if any benefit, unless you are stuck having it in one spot and there is a big peak or null right where you're sitting. 

There's also no need to "relieve" the speakers of the lower frequencies.  First of all, they only go down to 75hz, and I'm not driving them with a flea amp.  In another system, that might help, but not in mine, and without knowing more about your system, nobody can tell you that's necessary.

I don't know anything about your speakers and sub, but they look like they are designed for home theater use.  What's the rest of your system?  What about it do you currently find lacking?  What do you like to listen to, and how loud?  There are probably other things to consider than the size of the cone in the sub if you want to improve your system.

Sounds like a nice system for the room and if you enjoy it now, I'd ask again what you think you need to "improve".  You mention integration.  Do you feel like your current sub is not well integrated with your speakers?  Do you have room to experiment with placement?  Have you experimented with the crossover?  Phase?  If your speakers only go down to 100hz, have you tried a higher crossover (assuming you can)?  It's unlikely that getting the bigger sub is going to have any detrimental effect, so you can always just get it and try it if you have the budget.  You might consider a sub that allows you to crossover higher and has additional controls for PEQ, like Rythmik's subs, allowing you to manually configure the sub to your tastes.  The ease of use of the DSP option is nice though and could be just what the doctor ordered.