Over sizing rads is generally good, as at the same delta T you'll get more heat output from them. I've done the same.

However, your system needs to run at the delta T it was designed for. This design includes your boiler. No condensing boiler's heat exchanger could handle the stresses of a 35 degree delta T and would most likely result in premature failure. To prevent this, the boiler usually goes to an error code once the delta T exceeds a certain threshold (on Vaillants, this threshold is 30 degrees). You should balance the system to try and get as close to the boiler's design delta T as possible.