i think the best thing to do is pick on up at GC and try it again. if it's not for you, the exersize the return policy

I use an HD500 with a classic JVM410C in the 4 cable method (4CM).  I went for the HD over the M series because I wanted more than four concurrent effects and the flexibility of order and routing.  With the HD I can have up to eight of whatever, wherever I want them.  When using the HD for FX only I have never actually run into a DSP overload issue.  I guess if I stacked a  bunch of particleverbs I might be able to but within practical usage you should not see an issue.

Is it tranparent?  Well if I hook up in 4CM and listen very carefully then I can detect the slightest loss of shimmer on the JVM Clean green mode .  Does it put me off?  Well, no.  The tone is still really darn good and the benefits of my 4CM hookup far outweigh the impact of what I am hearing.  My focus is live rather than recording so my views are given in that context.

Noise was an issue on dirtier channels, in a 4CM hookup, due to a ground loop between the amp input and FX loop via the POD.  The fix for me was to modify my loop cables to remove the ground connection.  This is safe and killed the noise.  Bottom end loss ..... I don't think so but I have not really been listening explicitly for that.  If you do go this way ... let me know what you think as I am not much of a a size queen for the bottom end on amps .  The classic JVM loop was parallel and always leaks a bit even with the mix at 100% and that is not cool for tone preservation.  I think the loop is much improved on the Satriani model and is fully serial.  If you run the HD fully in the loop  for delays and modulations then you can easily knock it out of the signal chain anyway.  If needed, I have a method to knock the 'front' part of the hookup in 4CM out also using a true bypass box.  I don't actually use it  in practice cause I really don't 'feel' tone suck in the hookup.

Combine all that with the MIDI amp switching for the JVM, controlled from the Pod HD and there is a lot to commend it.  Marshall have dramatically improved the MIDI on the Satriani model and you can now individually turn features (master/fx loop/channel/reverb) on and off using Midi CC commands alongside the already-available full program changes using Midi PCs.  This means you could (for example) assign a footswitch on the HD to toggle to 'Master 2' on the JVM for a solo.  You can effectively now replace the JVM Footswitch with an HD500 or Midi controller.... there is full functional overlap from what I see.

Is the setup perfect and pristine enough?  Well that depends on your standards but it is good enough to meet what I need.  I would definitly suggest that you give an HD500 a whirl.   If you want to try 4CM, my guide was developed on a JVM and should be good for the JS model.  http://jvmforum.com/phpBB3/viewtopic.php?f=9&t=5333.  Must admit that the JVM410HJS is the first Amp that has given me GAS in a while (though I would miss my reverbs and clean green !).