Gordon I'd like to take a shot at answering your question concerning performance gains but without the carb step . My wifes 550 is fairly mild at 592, web 58a and stiff springs, balanced, porting improvements, tighter squish and 10.1:1CR, a pipe, "squared up" the cam timing and 106 LC's.
It's docile and very "streetable" which is what I wanted for her, and although it needs a touch of clutch to loft the front, wheelies are easy to ride on the throttle once up. It REALLY accelerates. It reaches top-end quickly BUT runs out of wind at 115 and 9200rpm on stock gearing. It needs bigger carbs and I think that improvement alone would really do something, but in this case it runs perfectly as is and my wife doesn't need any more! Even in "mild trim" IT FLAT OUT RUNS AWAY FROM MY 750 and it's a good one albeit mostly stock.
Besides all the other handling & braking improvement goodies, this bike lost a bunch of weight which is indeed part of the performance gain. I'm not sure what single improvement gives the biggest bang because I had decided from the onset that this whole bike would be re-invented and rolled out of the shop that way.
So compared to a stock 500/550 the difference is as you stated night & day. It feels tiny and is a blast to ride.