Symon,
First, you are correct that the power is spread over a wider frequency band by 'spread spectrum', and is in no way reduced (the same power, either way). It that sense, it is a sham, as the noise floor just rises. Or, if you have a wideband channel, the spread spectrum is just a real pain, as it is all there, audible (or visible), interfering. Most of the spread spectrum clock generators use a triangular wideband FM modulation, so they are really very annoying if your receiver is wide enough to hear it (it just howls). "True" ss would be a psuedo noise signal, which is less annoying, but still interference! These ss clock generatorss do "fool" the FCC and CE test setups into believing they are "not there." As far as a police or fire radio is concerned, they are not 'there', and will not interfere.
Second, Xilinx was always careful to note that specific implementations of PLLs might not deliver all the benefits that should be there, due to limitations with sharing common ground, and power, adjacent circuit interference, etc. Not that we are able to be immune from everything, but at least by studying the competition, and knowing the weaknesses, we could set the goal to exceed their performance (which we did).
Third, the "normal" spread spectrum is unable to cause a DCM to lose lock, or prevent it from locking (as you so rightly point out). We just did not want to have to characterize a dozen or so spread spectrum clock generators and our DCM to PROVE it. Since the 300 ps spec applies to the clock edges that are critical (which are 6 clocks apart in V2, V2P, and 36 clocks apart in V4, V5), it is tough to be absolutely sure that in 36 clock, 300 ps will never accrue. Are you sure? If so, then use our DCM with your spread spectrum clock.
Austin
PS:
Drove by a hybrid electric vehicle yesterday, and it completely wiped out a low power distant FM radio station (went from full quieting stereo music to loud hash and buzzing!). Cars do not have to meet FCC or CE rules. A real sham. (and a shame)