Even after 2 years or so riding single speed, I still find it surprising how little difference it makes to overall speed when compared to a geared bike. I've had a few futile commuter races where got overtaken going downhill but just take it back on the next climb or long flat each time. There is one chap I occasionally ride with on a commute who can take me whatever I'm riding so I usually just draft him! I'm finding fixed a little slower on the outward commute (downhill overall) but the same or possibly quicker on the return.
You look well on your way to your target and I've had a look at my mileage so far this year. Getting back on track to my usual 6000-6500 a year after a bit of blip in the last few months of 2010 when my daughter was ill.
Commute Single speed/fixed - 1760
Road geared - 203
MTB (mostly SS) - 307
Hopefully there'll be some decent weather so I can get out on the geared bike a little more often