I have a P7 D-bin and a C-bin, I also have a P4, The reason why I chose the P7's was for the large flood the 4 dies provide and I was looking into a Q5 for the great beam it can provide.
The Q5 XR-E is great for throw in a flashlight, where the size of the reflector is limited, but in a vehicle headlight, the reflector is much larger, so you can still get a good beam out of the P7's larger area, and you can get 3x the light.
If switching the two lesser lights to P7's and the main light to HID(35w) is the best then I would rather do that.
Can't say what's
best, but I'm pretty sure a HID conversion is the brightest option for the price on the main light, and I think the XR-E would be way too dim.
I have no knowledge of HID lights, so consequently they never came to mind. Do you have any bulbs in mind? and what price is the HID conversion talking?
I think the kits typically run between $40 and $100 on ebay, but I'm not at all sure. (Also, the typical kits I've seen are meant for a car, with two ballasts and two bulbs. So either you get spare parts, or maybe you can find a motorcycle or ATV kit cheaper...)
As to the bulb, ideally you'd find out what type of bulb your headlight takes and find a matching HID kit, so it would fit in the same socket with no special mounting considerations. Obviously, if you're looking at LED conversions, you're not afraid of a little fabrication or modding, but if you
can find a drop-in, that's easiest. If you can't find a correct match, then you'd need to look at the location of your bulb's filament, and the locations of the arc in the various sizes of HID bulbs available, to pick one that you can mount with the arc in the right place.
The only recommendation I can really make is about color temperature -- 4300K is generally the brightest, with output loss as you go lower (yellow) or higher (blue) temperatures. If you really don't like the 4300K color, you might go as far up as 5000K, but it starts dropping off steeply past that. (Not sure how it breaks going down, but 3000K is the only lower-temp "white" option I've seen.)