Sorry, my mistake... I meant the GR-20 is what I'm using.
Even with that, I dare say you'd still have the same analog -> MIDI -> analog conversion happening, it just happens internally instead of spread over the signal chain.
I've tried a GI-20 connected to a synth module and the tracking was much slower than my GR-09. And the GR-09 is an old model. The MIDI protocol is a modified RS232 so its speed is limited. While the GR-20 may have internal analog<->digital conversion, it's not the same as MIDI. Here's a quote from the GR-09 manual. "The response time (and other factors) can seem somewhat altered, and you may feel there is a slight disadvantage to using MIDI to play an external unit, as opposed to the GR-09's internal sound generator. This is caused by restrictions in MIDI specifications, not the GR-09 or your external unit."
In the early days of guitar synths, I remember Craig Anderton writing that a less lively sounding guitar possibly with old strings tracks better because the fundamental tone is stronger since less upper harmonics have to be filtered out. I have also noticed that my mahogany-bodied Ibanez 540SLTD with a GK-2A pickup tracks a bit better than my basswood-bodied RG1520 with its piezo-equipped trem. So I would think that a lively guitar like a Strat might not be the ideal candidate for a guitar synth, and a more mid-heavy guitar would be better, even though Roland may have improved their techniques for filtering out the harmonics in recent years.