You know, it's so funny that I'm seeing this question just now. I had a question about recommended clematis for growing on a lilac earlier today. My thoughts are the same as yours... planting one that isn't too vigorous so that it doesn't choke the lilac. But, the request I got was for a vigorous one.
My suggestions for something a little different were either Arctic Queen (a beautiful white that tops out at about 8 feet, has double blooms on a very sturdy stem, and provides great cut flowers) or Kingfisher (a deep purple blue double, also about 8 feet, with the same characteristics). Both of these are Pruning Group 2, though, so they would be lightly pruned... not hard pruned to the ground.
If you are looking for a Pruning Group 3 that you can whack to the ground each season with no worries, I'd recommend either Prince Charles (great bloomer, mannerly habit, very easy to grow, grows to 8 feet, light lavender bloom), viticella Alba Luxurians (another great bloomer, white with a very interesting green tinge to the edges), or texensis Princess Diana if you would prefer nodding hot pink flowers (also an extremely profuse bloomer).
