Yes, HUGE weight-savings going to 2-1. Check out this Area P pre-gen exhaust thread:
https://www.ninjette.org/forums/showthread.php?t=13852
Since it's already tuned for max-power at high-RPMs, more gains can be had in mid-range. Which is what you want on street-bike anyway. Apparently really significant gains in mid-range.
Check this out for merge collectors:
https://coneeng.com/collector_components.html
and this for tubing:
https://burnsstainless.com/collectio...ends-304-steel