1500 to 2500 is almost 3 dB, noticed only if needed though. The ultimax has usable excursion to 28mm and running it at 1500 watts in a 4 cubic foot box yeilds 22mm. So this design leaves more excursion on the table and not ideal for this driver. Now if all one needs is that of course it will work, and work great. The difference between 22mm and 28mm is bigger than the difference between the UM18 and the JTR driver at 33mm. Just sayin, limiting it to that makes more on par with the Si 18 and others with less excursion. I guess it depends on the person. They won't notice if they never try.
I have no experience with that plate amp. In general I would avoid them if at all possible, even if I am sure there are reliable plate amps in existence, as there are also those that are not, and if you've got a whole cut in your enclosure for particular one, and that one proves unreliable... I don't need that headache, and I don't need to test the reliability with a regular vibration torture test. My unreliable speculation is that it was more a combination of the plate amps having a more punishing environment to begin with, and that those particular amps are probably cheap junk with crap QC during manufacturing. But that's only unreliable speculation, so...
My experience with UM-18s is that they're not what PE promises (higher Qtc, faster/sooner roll-off than suggested by their recommended box sizes), but that's still better than most anything you'll likely find as a finished product, especially available at a retail store, for the price of a kit and a cheap pro amp, at least in terms of doing what it does well, giving you gobs of useable, relatively high quality low frequency output at a reasonable cost for the driver. To be fair, a DIY kit and a finished commercial product is not a fair comparison, at all.
4 cubic feet will function as a sealed alignment, though they will likely perform better, deeper, with less power if you give them more air. With 4 cubic feet, I did need to give them an aggressive shelving filter and 'transform' their roll off to get flat extension in-room below 20 Hz, though it was hardly a problem. Within my listening requirements, I never approached limitations that seemed like it was finding the limits of either the drivers or the amplifiers rated to 1000 watts each at 4 ohms. With those amplifiers honestly rated to provide 1000 watts, in outdoor sweeps from 10-300 Hz, at equivalent voltage, they got moving to what sure looked like full excursion, if not very close, and the enclosures were rocking and sliding on the moving blanket... Can it actually do that excursion at 10 Hz and a '1000 watt' equivalent signal output? I don't know. Perhaps not. If that was a priority, I might investigate further and the importance of more box kicks in. At 30-40 Hz, where they benefit from a higher Qtc system resonance through that frequency range, I would certainly think so.
With two of them, comparatively speaking, with proper DSP compensation and integration methods, compared to your other commercial options, they should be able to pound hard and dig low. I would mention, though, that if you're looking to use them for anything resembling a 'Hi-Fi' or audiophile application, count on about 12-13mm of excursion that is relatively noise/distortion-free to a critical listener. For me, in my setup, with my room, and my listening needs, with two of them, that was actually plenty. Once integrated properly, they sounded pretty good, and SPL, extension, power handling, power delivery, or excursion was not a limitation for my use. Even if I believe they would perform better with 8 cubic feet, I didn't want to have two 8 cubic foot enclosures in my living room, so I did the best I could with what I had, and eventually just upgraded drivers.
If YOU have the stomach/space and optimal placement location possibilities for two enclosures with 8 cubic feet or more of
internal volume, cool, and in which case there are probably a lot of other options available in a similar price range that others can give you much better information comparing and contrasting, depending on your priorities.