It has probably all been said but I can't resist my two-penneth. Weight training can improve ccling performance. So can running. So can swimming. So can walking upstairs. They all work better than sitting on the couch. But in my opinion it is highly unlikely any of them improve cycling performance MORE so than additional EFFECTIVE on the bike training. In a 10 mile TT you might do 2000 reps at more than one a second. I cannot be convinced that 10 or 20 rep stuff, one every 3 to 5 seconds, is more beneficial than on the bike training. if you aren't already doing 12 hours or more on the bike, you still have major gains you can achieve from being on the bike.