We cannot expect the SE2 guys to optimise their transmissions, we can only be grateful they have done something and purchased some EC.
And what about those of us who have put up ground stations at our own expense?
If the SE2 guys don't want to maximise their EC, why do we have to carry the can? Everyone else is optimising.
Those of us erecting ground stations might be doing a lot more optimising... and have made the optimisation possible.
If the SE2 guys don't optimise, their transmissions may not be received even by ground stations.
In the end all you can do is fly with a PilotAware and a SE2 on the same flight and note the results, in your aircraft.
I have but I'm suggesting you gather your own evidence.
Your main problem is that the PA28 is an all metal aircraft and will act like a partial Faraday cage for any carry on you have.
This is the main problem with any carry on, and it will limit PAW signals in and out, and SE2 signals in and out. This is probably the issue you have with not seeing much ADS-B.
Whilst I realise that you cannot have external aerials, I am in an aircraft where I can and the results are very different.
Rebroadcasting ADS-B upwards from ground stations towards the metal under surfaces of any aircraft is less likely to work than air to air, so the SE2 posse may be right when they say the air to air is better, but that's the obscuration of their metal hull at work.
Bottom line - there is a reason why a metal (or metal lined) apron is given to people having X-rays to protect their tender parts. The same metal "protects" antennas from receiving transmissions.