I dont know why the manufacturers rather just dont build a 50 watt PA into qrp radios, you can operate either at 10 watts variable, or turn the 50 watt PA on when you want extra power.
Because many QRP operators don't want to pay for something they will never use, or carry extra weight they will never use, because a radio that has 50w output doesn't fit the generally accepted standard of "QRP radio", etc.
Nobody would notice the extra 3db at 50 watts of output power.
That totally depends on condx, antennas, etc. If 3db makes a difference at the 5 watt level, it would at the 50 watt level also.
To me a radio like a KX3 with an external PA is a unruly mess that I dont need in my life, there are much better solutions available even if they dont offer contest grade receiver performance. I should also stress that you will hardly challenger any receiver these days operating with a wire or portable antenna!
Here we go again...sheesh!
My FT817ND's RX is definitely challenged using a portable wire antenna at times.