Similar to Grammy, I've had a WT Belay Jacket for about 13 years now and love it - but use it sparingly these days. Most of the time I take my Patagonia Micro Puff Hooded Jacket. It's synthetic (100g Primaloft One), just over a pound, packs down pretty small, and keeps me warm enough on all but the coldest belays (everyone's different though). Depending on the options you choose for the Insulight, it would be very similar.
If you like WT and want to keep it local, get the hooded 100g Primaloft One custom version of the Insulight jacket. If you're looking for other options of midweight (at least 100g) synthetic hooded jackets (and I agree with Dave that the hood is a necessity), check out the TNF Redpoint Optimus, or the Rab Generator Alpine. There are different versions of Primaloft, and those jackets are two of the few remaining midweight synthetic jackets that still use Primaloft One - which is the top of their line. They also have a good cut and feature set. (FYI - WT uses Primaloft Sport in the stock Insulight and even the Belay Jacket now, Patagonia uses Sport for the latest Micro Puff Hoody. It's less expensive but not as thermally efficient. MH uses their own proprietary insulation on the Compressor, I don't remember the name of it though.) There are several new synthetic insulations coming out next fall if you're not in a hurry, and water-resistant down is going to be everywhere it seems.
The last thing I'll add (if you haven't been bored to sleep yet), is that if you go with the custom Insulight, see if they will make it without the Powerstretch or WindPro side panels. Those fabrics are great for breathability and mobility - neither of which is needed for your belay jacket. You want it to be warm, durable, and wind PROOF. Ask them to make it with the insulation all around, like on the stock version.
Hope that helps.