I don't think its a CPU/polyphony issue.
The PreenFM2 does FM. Operator feedback in FM changes the pitch of the sound. I recommend using other waveforms to get the sound you want. For example, when I want to simulate an operator feedback sound I use the saw waveform. It works really well for the types of sounds I'm doing. There are also some user waveforms on the forum with varying amounts of feedback.
The Yamaha DX series actually uses Phase Modulation.
However, along these lines, what might be cool would be having for example, a new algorithm, something like Algorithm 1, but have a modulation index (IM4) to allow OP1 modulate OP2.