Maybe the material used for the stock bolt was cheaper, but they still found that it worked. In a $400 marker, they aren't going to put the best of the best materials into it, even if it is a reputable company like Eclipse. Hence the composite parts. Although they're still strong, it's still cheaper than aluminum. There will always be room for improvement.
The stock bolt is titanium. It was used because its the right weight. The cost to make a bolt out of titanium is somewhere around 6-10 times as much as aluminum. Its more expensive raw material that is MUCH harder to machine.
I've been testing bolts for about 6 months now and have yet to find a better solution than the stock bolt.
I've got various bolt designs I've made with different materials (aluminum, brass, steel, etc) with weights from 20 grams up to 40 grams.
Currently my most successful design is an aluminum bolt with a brass core. the brass makes it slide like warm butter, but the trade off is a reduction in how well the antichop works.
Unless TechT has done something I just can't wrap my head around, a light etha bolt = more chops due to greater bolt speed.