That eBay item appears to be a narrow belt conversion kit, that lets you keep the stock fan & generator. Is that what you are looking for? The stock wide belt components are readily available, albeit separately instead of a package (since most people only replace the defective components).
Another point to consider is whether you want to keep the stock fan & generator. If you are planning to go to an alternator and/or electric fan, it might make more sense to buy a compatible kit rather than buy this one and then have to make further modifications. I'm not saying you can't do it the other way, it's just extra hassle and expense. I believe Moss sells a kit that is already set up for electric fan and alternator.
Don't expect any noticeable performance improvement from this conversion. The water pump and generator pulleys are no doubt lighter than the stock units, but the stock ones aren't all that heavy and 50% of a small amount is an even smaller amount. In other words, shaving a few ounces just doesn't make much difference. Losing the stock fan & extension will make more of a difference (and even that is just barely detectable by the seat of your pants.).
Durability would also be a concern, for me. The water pump pulley is cantilevered back so the belt is more or less centered over the bearings. That reduces the load on the bearings (for longer life) but puts a lot more stress on the joint between the pulley and the shaft. The pulley has to be a very snug fit on the shaft, or the joint will eventually fail (which is no fun at all). And since aluminum expands faster with heat than the steel shaft does, there is at least a possibility that the pulley will loosen up when the engine gets hot. Not necessarily a problem, just something to think about. A lot of these 'performance' kits are aimed at racers who will never put another 10,000 miles on the engine and hence are less concerned about long-term durability.