How much have sub-woofers changed over the years?

The Rel is a tighter more musical sub, yes. I've sold both retail over the years. However, placement, setup, calibration and tweeking is going to amount to FAR FAR MORE than simply weighing one sub over another in this case.
Personally, I like the Rel's for more subdued music dubties, as with the F1200 really. Both can be overloaded and or shut down if you drive em too hard for movies.(also, how big is the room/space).
If you're looking for improvement, where are you looking improvement for? Are you trying to improve output and dynamics, or are you looking for more accurate musicality.
Dunno anything about your situation really. But I'd recomend a sub that does supperb double dubty for both music and HT for a song! And that's the Paradigm Servo 15. Also, the M&K MX200 and above, but they're pricey for what you're getting I think. Really, the MX350 is more a closer match for the SErvo15, or maybe the MX5000 M&K sub.
I really don't see the hype on these Rel Subs for serious HT applications, only more subdued music listening. I think there are better HT/music subs out there.
All that being said, 75% or more of your performance and effectiveness of your sub is going to be placment and finetuning FOR CERTAIN! This is where everyone fall short mostly.
So, to answer your question about changes in sub's, I'd say they haven't changed enough to cause a revolution or anything. Bigger subs can belt it out easier still, while smaller subs need more watts to try and keep up(often at the expense of accuracy). Also, subs with inherently tigheter "Q factor" are usually more musically accurate, while more dedicated HT subs often sacrifice accuracy for output. Still, with the advent of servo's and such, nothing has really changed. A good push pull design, sealed acoustic design, a well integrated bandpass design, transmission line, you name it, all can be designed with very good effectiveness for what's needed if DESIGNED CORRECTLY!
The Rel's are good subs, but depends on the application from my experience. I've sold Rel's at 2 high end stores now, and know what they can do, as with other brands.

I'm running a velodyne ULD-15, 400 watt servo with brand new soliloquys and couldn't be happier. All that has changed over the years is that Velodynes have gained more wattage and performance for the money. A used sub for probably 1/3 the price is a great deal and an awesome performer!

I have a Velodyne F1800X (750watt version of the F1800) on my video system and placement and configuration really does make a difference (I put mine on a Sistrum SP4 stand and the bass tighened up signifactly in my system). But if you are looking for more output! Get another 12" Velodyne and go Stereo subs and place them on the same wall (not next to each other but othe either side of your center or corners..) This dual sub configurations helps cancel out nasty room nodes... Plus Stereo bass rocks! (I use to have 2 VMPS subs but the size made me get rid of them...)

I use a REL Strata II for a SET tube system and also have used it with a dedicated home theatre system. It works at least as well as the Energy subwoofer (like the top of the line Sunfire 1000 watt sub; small things with big performance) on home theatre stuff and excels at integrating with the two channel system speakers which are Galante Rhapsody high efficiency monitors.

If you want real home theatre noises find a used McIntosh passive dual 12" subwoofer and use it in conjuntion with a very powerful solid state amplifier. A neighbour uses that on his X.1 channel and it moves mountains.