Time for an update on this. Mostly in the vain hope it might possibly benefit someone else one day!
Quick recap of the initial issues.
Agent deploy tool hangs for 4 minutes at each click of a button:
Still the same, I just don't bother using it. Install Agent from the client end.
HP450G3 machines on Win7 Pro x64 SP1 take 17 minutes to show as active in the console after client reboots:
Again, no answer as such, in the end, I managed to recover a very early image I took using our old GSS2.5.1 then using that, make a set of product recovery discs, built a machine from there and what do you know - no delays. I spent a good while checking driver versions and making sure they matched but no luck. Concluded there must have been some chunk of HP installed bloatware left lurking that I couldn't find that was causing the trouble. Gave up on the as-supplied WIn7 build and am now progressing with the new recovery-disc based build.
Even this is a bit weird....having got a build fully updated I figured it was a good point to declare that the 'base build'. Bear in mind I had already created an image from the freshly recovered machine and deployed that back & forth a bit to othe machines just for a sanity check. Tried to create a new image, only to find the machine won't boot to automation, throwing an error in Boot Manager....but not the usual 0xC0000001 'A required device isn't connected or can't be accessed', this one was a 0xC0000003 'an unexpected error occurred'. Messed with that for a bit then to my great surprise, found that PXE now works..... My suspicion here is BIOS version as at the moment, I have one on V1.12 and the rest on V1.11. It's the V1.12 that will now refuse to run the locally stored AE but WILL now run the PXE delivered version of the exact same environment.
Trying to deploy this image back to the machine that I originally recovered, and used to generate the 'core' Win7 image and it tells me the disc is too small and try using -FIS (which didn't work). WTF???
PXE delivered WinPE to an HP450G3 just results in a bright blue, blank screen, then eventually a reboot:
Curiously, as mentioned above, V1.12 BIOS seems to have cured this....but broken the ability to run the AE from the locally stored version. On balance, I think I will opt for the earlier BIOS as the new one seems to be giving me other isseus as well.
Finally, the period where I was unable to deploy images that previosuly worked.....well, still not clear on what happened or why, but after rebuilding the machine using the recovery DVD, it now works again, I can again deploy images the originally worked, then failed.
I feel like John Snow...I know nothing. Well, not entirely true, I do know that I only have to tolerate this 'product' for another 2 months and then I am out of here. I so wanted to leave a reliable, stable, working solution for my soon to be former colleagues but this 'product' clearly isn't that. It would be easy to blame the HP450G3 machines. After all, our other plaftorms work fine, BUT, this is a modern platform and GSS3.1 is allegedly an up to date product so what's the damn problem.
OK, rant over, best advice I can give, avoid HP450G3 machines Win7 and GSS3.1 in combination.