Good thought Max; no worries though about burned valves or dropping valve seats with just an exhaust. Did you happen to see the video in post #107?

Manufacturers do lean the AFR... but only at idle and low load conditions. You'll see 14.7 on just about anything new-ish on the road. Under load, however, there is simply no getting around the need for a richer mixture. From the few dyno sheets I've seen, GL1800's are dialed in PERFECTLY at about 12.5 at WOT (wide open throttle). Exhaust work won't change that; you'll need to alter the intake side for that.