Tomtom used to get a bad rap for reception with app 1.0 vs the competition.
My guess is that app 1.1's "GPS signal improvement" meant Tomtom started listening to Skyhook (iPhone's wifi hotspot location registry) and perhaps AT&T A-GPS.
In the downtowns of cities, satellite reception is pretty horrible. My Tomtom 720 has the same Sirf3 chipset as the Tomtom mount, but it performs horribly once nearby buildings get above 20 stories. I'm not sure if the EPT on a 920/930 fixes that because it doesn't lose signal, it just puts you on the wrong street because of the building reflections.
For the big downtowns, you need a backup to satellite, even with a top-of-the-line chip like SirfStarIII. Either a built in car GPS that can use the wheels as backup, or a backup local technology like Skyhook.
My suspicion is that in dense urban areas where most iPhone owners live, the iPhone Skyhook/AGPS backup tech is so good that that a SirfStar doesn't help. But take an iPhone into the interstate (especially rural mountanous windy roads) and my prediction is that the Tomtom mount will shine.