I'm interested in trying to push my QCX into being usable on both the 20m and 30m bands, with some obvious reductions in efficiency. The parts that would affect the performance seem to be the LPF,John,
Hans makes his RX kits quite narrow compared with other SDRs that use similar circuitry.
The Softrock RXTX for example had a version covering 40 - 30 - 20m with only the need to add a LPF for 20m.
I do not think you would find a wider BPF in the QCX RX would make any great difference. Nor would the small difference in the windings on T1.
The PA is a different thing. It is designed as a single band class E. I'd imagine that the 20m LPF, again tighter than Softrocks, would still be adequate at 30m although that would need testing depending how concerned you are.
As for optimising L4 and C30 for two bands I guess reasonable results could be achieved.
In view of a few reports of catastrophic failure due to PA breakdown I would suggest isolating IC3 pin 3 with a capacitor. Use a resistor to keep the gates low when not driven.
73 Alan G4ZFQ