Hi Tim!
Some years back I went through a similar thing. I wanted to put HID's in the bike. It turned out to be a bit of a journey, here's what I remember of it
I found what looked to be great ballasts, they were also slim units looking very similar to the ones you linked to. I really wanted to go 50 watts on the high side so that I could have the ultimate in brightness
The 50 watt high beam ran too hot - fortunately I caught it in time. What happens with 50 watts is that it begins to turn the plastic headlight lens assembly golden colored. So 50's out of the question right off the bat.
Not deterred I carried on with the 35's. Problem was, they flickered. It was not very appealing. I then did experiments with my other ballasts and even bought the 'anti-flicker' box (an extra cost option - probably an over-sized capacitor) and it still flickered. At that point, I said forget it. I had to install a new wiring harness (to do it properly so that they lights were on a switched relay running directly off the battery), I had to install the anti-flicker option and in the end, same thing.
It wasn't a big pain in the rear but pretty soon I had a lot of wires running fore and aft plus two ballasts and still....flicker.
Even though I quit the conversion I did some more research on it and what I found out was that the really good ballasts are made by Phillips and they cost a lot. They don't flicker and they are the real deal. Impossible to get and very expensive (at the time, like $500 ?).
And then there's....the warmup time. HID's take a few moments to warm up and so, if you think of one on the high-beam side, you flick the switch to go high-beam and....a few seconds later your light comes on and then begins to get brighter....
They also don't like to be power-cycled like that. They normally are started up and then remain on the entire time the engine is operating (like the low beam does).
What I like about the HID's (other then that they are very interesting and cool is that they use less power - 35 watts as opposed to the 55 the stock bulbs use). In the few years since I tried out the HID experiment LED has come into vogue. I've not looked at them at all but it might be worth a look-see to see if they have come of age in some meaningful way.
I love HID's as a concept. I did find a company that has good HID's that were guaranteed not to flicker and knew all about those issues. They knew what they were doing and designed their product line around them. For the life of me I can't remember their name but the pricing was just over $100 USD for a single bulb (one ballast & bulb).
The other thing I recall is that most folks chose the wrong bulb. If I remember correctly you want something in the 5700k range and everyone was going with really odd ranges so that they would get different colors. The problem is that brightness drops off dramatically as you go up the kelvin range so they were shooting themselves in the foot.
In the end I took a much smarter approach. The headlight system on these bikes is very well designed. The beam is focused very precisely, so the Low Beam only raises up to a certain height and then it's cut off completely. The High Beam is designed to start working at that specific cut off point and then rise up. So I changed the wiring in the switch so that when the bike is on high-beam both lights are active. The OE design is one or the other. By going with both you get a full spectrum light from ground-level right up to the top as both are active.
I've never had an issue with the light output on the bike. I regularly ride late at night through forested areas at speed, canyon carving, etc. I've been really happy with the lights. I do remember that a fair number of folks installed GE NightHawk bulbs and were really, really impressed with them.
I looked into that and found out there are two (?) versions of the NightHawks - you want to the high end ones. My guess is two of those are probably close to $100. I'm guessing that the best system is probably what I have now (both lights on when on high-beam) but with the really good NightHawk Bulbs (I think they were called Platinum?).
Keep in mind though, with both bulbs on, witch is NOT OE design, the lights will draw 110 watts and that will likely mean that at idle if both lights are on, you may be running the battery down.