Well, I can now answer my own question since I went and got a DRX595 to play about with and I've got to say I'm impressed. In case anyone else wants to know here are the results.
I hooked up my newly acquired Sky DRX595-C to my parents 1.8m dish in Helsinki to see how it compared with the old ex-pats popular Pace 2600C1 for Astra 2D reception. The DRX595 just has the edge for reception. Some of the weaker BBC 1 regions where coming in with less breakup than on the 2600C1 but the big improvement was with the audio. The 2600C1 tends to peep and squeak in an annoying way quite frequently when there is slight picture breakup but the DRX595's audio was almost prefect with just the very occasional dip in sound, but no annoying noises.
I then hooked it up to my 80cm dish and compared a few of the weaker Eurobird and Astra 2A/2B south beam channels, almost every time the DRX595 beat the Pace 2600C1 on the weaker channels. On a couple of channels I couldn't tell much difference, breakup was about the same, but the audio was always better on the DRX595.
I've attached a couple of screenshots with my TV showing both screens at the same time so I could get a good real-time comparison of both boxes being fed off the same LNB. (And to make sure one LNB output or cable wasn't giving a better signal I swapped the feeds over and compared them again with the same results)
The screenshots show:
CNN - A good example of bad breakup on the 2600C1 and perfect on the DRX595.
Sky News - Too weak for the Pace 2600C1 but the DRX595 was trying to get a lock, even if it wasn't watchable.
Bloomberg - Both boxes showed about the same amount of video breakup, but audio better on the DRX595.
I also compared the DRX595 to my Technomate 6900 (older non-super version) and the DRX595 easily won hands down. (The Pace 2600C1 beats it too)
Conclusion: The Sky DRX595 isn't going to perform miracles, but it does provide less breakup on some channels and the audio quality is a big improvement so giving you slightly more rain fade margin or perhaps a bit of extra time on a 2D channel if you don't get it 24 hours compared to a Pace 2600C1.
