Thank you for all the replies so far. This evening I fitted a pair of Philips X-tremes to the dips. I started with one side only, so as to compare. Initially, close to, I thought the X-treme was much brighter than the 100W, but when I walked ahead of the car and looked back I found that the X-treme beam was now pointing higher (too high) than the 100W beam. Adjusting it down to the same level and standing away from the car looking back there didn't seem to be much in it, if anything. So I would say that the 55W X-treme is as bright as a Ring 100W (which is an achievement in it's self), but it still wasn't as "night & day" better compared to the original standard bulbs as I had been hopind for - perhaps I was expecting too much.
Having left the car running for a while with the dips on, I then compared the external heat of the "glasses". I found the X-treme to be very hot at the hottest point, but only at the hottest bit, whereas the 100W was very hot over a larger area.
To me this would imply that the X-treme is somehow better concentrating its power - perhaps this is part of how they get improved lighting out of 55W?
I then fitted the second X-treme (I'm getting the knack of changing these bulbs now!), adjusted it's beam down a little too, as necessary, and took the car out for a run. It was again as good as the Ring 100W's, but I would not say it was better. The Ring bulbs themselves showed absolutely no sign of having been getting hot - and there was no heat build up in the wiring, behind the bulb or elsewhere.
So at this point I have still got about the same (definite, but not "night & day") improvement over standard as I'd had with 100W's, a bit less of a worry over the heat of the "glasses", and what little worry I had over the wiring has been entirely removed.
However I now actually NEED to put X-tremes into the main beams too since having had to lower the beam settings for the dips I have thereby also lowered the beam settings for the mains, which no longer point so far ahead... So on main beam I'm probably slightly worse off... I am hoping that putting X-tremes in will overcome this - ideally pointing that little bit higher like the H7's did.
Thanks again to all,