Actually you have the edge. The 98-00 SC400's engine was modified with a new vvi-T system that makes it put out now 290-300 HP and 300 ft-lbs torque. Lexus timed the new engine and car at 0-60 in 6.5 seconds. I've personally gotten mine at about 6.2-6.3 seconds. Autos.com rates it at 6 seconds flat.
http://www.autos.com/autos/car/luxury/2000/lexus/sc400/base
As for the E34 BMW 540i, it was rated at 0-60 in about 6.2 seconds in manual and 6.9 seconds in auto. If your friend has manual, he will have a slight edge, but in automatic BMW vs. Automatic Lexus, the SC400 is bound to win. I'm sure if the SC400 had ever been produced in manual, it would probably have gotten the 0-60 down to 5.5 seconds or less. Remember his car puts out slightly less HP and torque than yours, plus his car weighs much heavier and has a worse drag.
Whoever is telling you his BMW is better than yours has no sense of stlye whatsoever. In terms of sheer design, the 1st gen SC is better than a BMW anyday, even the new ones and especially your friend's old piece of junk. The SC was so pioneering when it came out. Your friend's BMW is nothing more than an unreliable German box of metal, in my opinion.
Don't forget to remove your spare tire and anything else in the trunk, 1/4 tank of gas is all you need to lighten the car up a bit. And of course remove anything else that isn't bolted down.