Judging whether a given bike is going to make you faster is pretty difficult on paper. For instance, one would think I'd be faster on my Moots, as it is lighter than my steel Bianchi, but I'm faster, by a fair amount, on my Bianchi.
Why? I dunno. I'd venture a guess that the Bianchi fits me better and allows me better utilize my major muscle groups, but given that we never really could nail down what wasn't working on the Moots, it's difficult to say. Maybe it's in my head. The minute I get on the Moots, I turn into an unhappy camper.
If I'm reading Soma and Surly's bike descriptions correctly, the Soma is lighter by a couple of pounds than the Pacer and your LHT. Will that make it an appreciably faster bike? Maybe. Maybe not. You'd likely see the biggest different in how "fast" the bike climbs. None of the bikes you're interested in, however, are what I would call "light," so it's all relative.
Your speed on the bike will likely have more to do with the tires and wheels you use and, more importantly, how you set yourself up on it. Saddle height and fore/aft position have a big effect on how you utilize your glutes, quads, and hamstrings. How upright you will also have an effect on your aerodynamics.
Of course, it bears saying that training is the most important factor in speed. I actually think you'll get faster if you train consistently with that purpose, regardless of the bike. I didn't get faster by an serious degree until I started forcing myself to ride faster--by chasing after people who were faster than me. Between your work on endurance and climbing, however, I think you already have a lot of focus on right now. Give it time.
I'd also add that you might get more money for your FX if it's already built up. By the time you pay somebody to strip the parts off, it might be a wash at best. I'd run the numbers to make sure it's worth selling the bike without the parts. If it were me, I'd want to save up for better componentry if I was buying a "go fast" bike. Of course, I like drivetrain bling, so keep that in mind.