Been following this thread (and chipping in my 2 penn'orth) and this has led me have a good look over the hid50.com web site.
Lots of info - the installation section look pretty detailed and clear.
Also, the comparison of lumens (lm = measurement of light output) clarifies my earlier post;
1250 - 1400 lm : 55 watt standard H7 halogen bulb
1500 - 1650 lm : 55 watt performance H7 halogen bulb (Philips 50+, PIAA 55=110 etc)
3000 - 3200 lm : 35 watt HID conversion
4600 - 5300 lm : 50 watt HID conversion
Rough maths taking the mid-range points shows;
Performance H7 is 25% brighter than standard H7
35 watt HID are 130% brighter than standard H7 and 100% brighter than performance H7's
50 watt HID are 280% brighter than standard H7 and 150% brighter than performance H7's
I'm now sold. hid50 kit ordered.