Which one are you looking to convert? Before I go on with answering this, I'd also like to ask what you plan on using the hub for--are you just putting in on a fixed gear bike or will you actually be using it on the track?
I would definitely NOT touch the G1 PowerTap if I were you. They had problems with moisture and the thing weighs well over a pound. It would also be wired and Cycleops does not service them anymore. Don't bother with it.
Up until a certain point PowerTaps used a 12mm axle, but they recently were converted to a 15mm axle, which increased stiffness and decreased weight. This change came later in the G2 hubs. There was also a change from steel axles to alloy ones which also helped in the weight and stiffness departments. All new Cycleops hubs come with a 15mm axle, and if you were to go that route you could get their dedicated track hub. But, at just over a grand in price I don't know if you'd be willing to go that route. That's where the conversion looks like a good deal. Although it depends on what model of G2 you get (Pro, SL, SL+), as these would all turn out differently in terms of weight and electronics (wired vs wireless, and coded wireless vs ANT+).
Ultimately, it depends on what you're trying to do with the wheel. If you're trying to make it fixed just because you can get one for cheap, you'd be wasting the potential of the hub. If you're trying to train with power on the track, saving up a little and getting the track hub would be a great way to go, and would probably save you a lot of hassle, but it might be overkill if that's not what you're going for. The conversion looks pretty good and well priced, but like you said you'd have to buy the hub as well. In any case, it sounds like a neat little project. Have fun