Motor power is important but internal gearing and energy transfer is probably more important. A solid metal gear set will sew more layers than a plastic gear set over the long run, no matter what the motor power is.
I can't imagine having a machine that doesn't do the job I need it to do. My 30 year old Bernina 1031 is still doing its job and can sew 4-6 layers of denim. My newer (12 years or so) Bernina 440 can handle multiple layers of batting and fabric when I sew quilted bags. If your machine isn't doing what you need it to do, see what your service tech recommends. Maybe just replacing the feed dogs or an internal part is all that is needed. If the machine is just so worn that it won't do the job, it is time to look for another machine (either new or used).
Pam