One of the reasons that the Mastr II (Exec) was chosen for the ProgRock tests was precisely because it is phase-modulated:  This makes it agnostic to the signal source since it is not required that the source (the ProgRock, in this case) be modulated.

In our assessment, in an "A/B" test, we found that the ProgRock versus Quartz different was minimal on VHF, very difficult to tell between the two in terms of phase noise (e.g. background hiss) on an unmodulated carrier.  At UHF (70cm) the background hiss was more noticeable, but we quickly realized that it depended on the radio used:  Some HTs were phase-noisier than the ProgRock-originated signal.  One can check this onesself by simply programming the ProgRock to the intended crystal frequency and tuning into the would-be harmonic had it actually been used in a radio:  If, after passing through multiplier stages, it is noisier than the "raw" ProgRock's harmonics, something in the multiplier chain is unhappy.

Our assessment was that base on phase noise alone, it was just fine on VHF - and it was probably fine on UHF, too - as long as one didn't have a string of series-linked repeaters, each with a ProgRock as its main reference.

As far as overall cleanliness, the ProgRock produces more low-level spurs than a crystal alone as one might expect:  A tight L/C band-pass filter may help, but since there are already such things within the radio's exciter, anyway, it may not be worth the difference.  When we looked at it on an analyzer at the 600 kHz and 5 MHz points (for 2m and 70cm splits, respectively) it looked "clean enough", but more scrutiny would be required if it was used on a shared site.  To be sure, we would be certain to use a true band-pass cavity since almost none of the so-called "Bp-Br" duplexers actually have true-bandpass response - but for lightning protection and good-neighborliness, we have always done that on our club's mountaintop sites, anyway!

73,
Clint
KA7OEI