Not necessarily. I know a horse with a roach back worse than hers, and he is out doing low level eventing (Pony club grade 4 I think, so around 60-70cm? Sorry I'm not familiar with pony club levels and protocols) and some basic dressage. He has not had a problem with his back as far as I know. As long as you are aware of what you're doing and tuned in to picking up any soreness, she should be fine.
As I said, it doesn't mean that she is going to be hopeless at eventing and fall apart if you jump her over 50cm, but when you compare her to a horse that is built for the job and bred for the job, both you and her are going to have a harder time reaching the same level.
As long as she's fit enough and doing enough flat work to continue building hind quarter strength, then keep doing what you're doing.
It will be very important for you to work very much on getting her swinging her back, engaging her hindquarters and working into the bridle to develop strength through her back and quarters.
Thats what we are doing!, my instructor has given us lots of hind quater strength builiding exercises and also things to do to get her working round and using her body correctly in the probably 1 and a 1/2 months I have been doing this we hve literally seen her muscles grow and develop, her hind quater is much less slanted now that it was not long ago and she is developing topline (i know this because we had to change her saddle gullet because she had grown more muscle!)