In fact, I've posted exactly that bit about a Li battery boosting performance several times - all in jest. I also point out that the bike will be easier to pick up if you drop it - also as a joke. But of course, there is an element of truth in both of those statements. Practically speaking, I doubt one could measure the increase in performance unless all aspects of the test were automated - and even then variables might make the results meaningless. As for a 25 lb or even 40 lb lighter ST having faster acceleration or a higher top speed that a weightier example, again, I doubt the difference would be enough to consistently show up in say, a drag race or top speed run. Operator reflexes, clutch actuation, and wind resistance (operator height and breadth (latter because we have a few, ahhh portly owners) influencing the last) would be more important.
Remember, these bikes are sport touring, and while a few guys have done race days, I've not heard of anyone consistently racing an ST, like any number of other vintage bikes. I remember going to MidOhio Race Course and watching Vintage Moto Guzzies or BMWs racing, but again, not STs. I'm sure these bikes could compete, but I think you would have to trim far more than 40 lbs. Some of the Guzzies I saw racing had pulled the starter and other nonessential 'accessories'.