New wheels or new bike?

What would you do? I am looking to make my ride a bit faster and zippier. I currently ride a Soma Smoothie ES, steel bike with steel fork. I have Shimano 105 components and my wheels are handbuilt- Mavic Open Sport rims with round steel spokes (don't know what type), Shimano 105 hubs and Schwalbe Durano 700x25 tires. I commute on this bike, do club rides, and very long unsupported rides (60-100+ miles). This bike fits me like a glove, and my Brooks B17 saddle is perfect. The trouble? People keep being amazed that I can ride such a "heavy" bike on the club rides (I am always the slowest, even downhill, but that's probably more me than the bike). In truth my bike stripped down (no fenders or rack) probably weighs 22-23 pounds. Getting a new carbon bike would be fun, but it's not exactly in the budget. My dream bike would be the Orbea Diva, although I haven't ridden it yet, I just love the way it looks. So that's in the 4k-6k range. I did test ride a Jamis Xenith Comp and a Cannondale Synapse 5($2100). The Jamis was not the right fit but the Cannondale was sweet. Both had 105 components, which is okay, but I would like Ultegra because it seems like if I'm going to get a new bike I should move up a level in the components for weight.
What might be more in my budget is a new set of wheels and a carbon fork. From other posts I've read I decided on a set of Velocity Aerohead rims with White Industries hubs and lightweight round steel spokes. OF course new wheels will also mean a new cassette because I would still keep my Soma as a commuter bike. So these upgrades to my current bike would cost roughly $1200. I value the input of everyone's opinions on this forum so I wonder what would make the most sense. Would new wheels and a carbon fork lighten up my ride,make me a little faster, and make going up hills zippier? Or am I deluding myself that these changes will help and should I save up and get a carbon bike. I do like the idea of just having one bike that can do it all and getting a bike that I would just ride during good weather seems decadent to me. Yet I can't stop drooling over the carbon bikes.

Just some experience replacing a steel fork with a carbon one (on our tandem).
It took 1kg off the bike, so that was a huge plus and it really helped make the road buzz less. You may want to look at a different rake fork it you find your bike is a bit hard to steer (ie takes a bit of an effort to turn or the turning circle is large) otherwise try and get exactly the same rake as your current bike.

Can you mount thinner tyres on your current wheels? Going down to a 23 might make a difference too.

I would test some lighter bikes on longish rides to see if that helps. We've been looking at Smoothie ES's for club-type rides, so I'll be curious what the other say. I can tell you that I was smoked on a ride last weekend by a lady on a Speedster (the lugged Soma frame). Didn't get much of a look at the components, but she had a rear rack with a tupperware container bungeed to it. Wow!

CA

+1 on skinnier tires as a cheap way to start.

I upgraded the wheels on my Lemond Reno to RaceXLites with Vittoria Open Corso tires - I got an immediate 10% improvement in my pace. The wheels were a few hundred $

I upgraded the Lemond Reno to a Madone 5.5 and got a 7% improvement in my pace (using my new wheels on the Madone), but the improvement took months to adjust to the new geometry, the compact double, etc and cost a few thousand $.

If you like your bike, you'll get the most pick up from reducing spinning weight...and may get the most zip for the $ and you can always move the new components to a new frame if you upgrade the bike later

Good luck

Thanks for the replies. CA, your reply really solidified things for me. What if I got a new bike and I was still the slowest. I better work on myself and my speed. I am still strongly leaning towards a carbon fork and new wheels though. It would be nice to have the bike lightened up just a bit.