On balance I think you are right Angelo. Right at least to the extent that the SBAS info cannot be properly processed by 3xxx series imo. But obviously my converted 3790>3490 can 'tune' sbas sats. I've seen PRN 153 and 193 with decent signal strength, but with conflicting results. With 153 showing, the accuracy seemed worse, with 193 it was better, (apparently). But this was maybe just random and the presence of the sbas signal was simply because the unit was unable to get enough signals from true gps sats so it locked on the sbas sat in desperation, who knows? Much more testing would be required to test that theory and prove one way or another if it can actually use the signal info. The fact that it can receive the signal is however indisputable. My layman's guess is that sbas is a 'red herring' and the real answer for all converts lies in modding the fw because the real problem is fw/hw partial-incompatibility. In my convert checking Predicted EPH and Cold Start then following Loly procedure resulted in faster lock and apparently improved accuracy. As dasilvarsa points out, this is about 'hotfix', and is predictive to give the unit a 'heads up' on where the sats should be. So to a certain extent, it's illusionary 'mirrors and cigar smoke' and is not really about accuracy. But in some units, accuracy 'apparently' improves. I say apparently because maybe that's illusionary too in that i don't really believe that original 3490 and 3590 really can maintain constant 3m accuracy, and my restored 3590 attained constant 3 m with fast lock after the procedure was done on it and it didn't demonstrate that before.
Summing up, this is what i think:
1. SBAS/WAAS is of no consequence and merely incidental. It happens because the 34xx convert is sometimes not able to lock to sufficient true GPS sats, so it picks up a SBAS sat signal but really can't use the broadcast info.
2. Setting Predicted EPH and Cold Start helps some units to show fast lock and improved accuracy, but it may not be really helping accuracy in those units in fact, and doesn't seem to help accuracy in others at all.
3. The real problem (especially for the unfortunates with 15-20m) is because of partial-incompatibility and the only way to fully overcome that is by a fw mod.
Just putting it out there as usual. Simply playing 'blind man's bluff' with no scientific backing.... but my money's on modding fw. This needs more brain-storming, where's Loly? ;)