If the SDX15 is also XBL2.... Those motors are glorious.
The SI 18HT has 5.3 liters of displacement and the SDX15 has 4.7 liters of displacement. By going from a pair of SDX15 to a pair of SI 18's, you go up 1.2L (12%) from 9.4L which isn't insignificant, but also isn't a huge amount.
With those numbers, I would stick with the SDX15+PRs unless you go up to 4x SI18HT. ..but only if you find the SDX lacking substantially.