g'day Peter,
28mm Ultra Gatorskins are definitely available in oz, I've used them quite a bit on my Club Tour.
Lately I've been using Panaracer T-Servs. These are a folding tyre, allegedly aimed at the bike messenger market. They seem to be a bit tougher than the UltraGators, particularly against sidewall damage, and seem to weigh about the same.
I think you will have to suck it and see, with the 25mm tyres. I'm not sure you will see much improvement, but the only way to know is to try. They may work better on the wider rims than John's experience with Audax wheels. The investment is a lot less than new rims, anyway. If your Audax rides include any dirt road segments, the 28mm tyres may be faster. The 25mm tyres may work better on the wider rims than John's experience with Audax wheels. The investment is a lot less than new rims, anyway.
I changed the Tekro cantilevers for Shimano BR-R550 brakes with Koolstop pads, and they work pretty well. Still squeal ferociously at times. I'd change the pads on your brakes for Koolstops (salmon or multi compound) and see if that helps. My experience is that the adjustment of pad alignment is fairly critical, and the Shimano setup works fairly well.
How do you find the Velocity rim? Is it less prone to shuddering under brakes than the Sun rim? My CR18s are all a bit misaligned at the pinned joint, and tend to shudder as the joint goes through the brakes.
Blair