With Starke SW-15 one would get half performance for half price - cheap build and aesthetics, half amp power, half SPL, much higher group delay after 40 Hz.
I'm sorry but no. CEA-2010 burst from AH testing shows same output at 20 Hz, 25 Hz, roughly a 1db advantage for REL at 31.5 Hz, and 3dB more for REL at 40. Presumably the higher amplifier power comes into play above 35 Hz, and the REL does show more sustained output, being able to maintain 100dB at closer to 24 Hz as compared to 27-28 for the Starke, again I presume because of the bigger amplifier, but for burst sub bass they are exceedingly similar.
And if you want more grunt above 35 Hz with the Starke, stack two and gain 6dB across the board, now obliterating the 1510 in deep bass.
Group delay might be lower, but I'd argue we're well within inaudible limits for both, as neither exceed one cycle ever. Distortion profiles for both show good behavior when pushed, for sealed subs.
You're paying for build-quality, aesthetics and brand with REL, as well as slightly better sustained output capabilities, and that might be worth double the money to some buyers.