The G5 includes a lot more features as has been mentioned but they are mainly intended for more heavily modified cars. For tune only type cars I think the main appeal with the G5 will be its new isolated boost control system. It features stronger and more linear throttle response and improved boost targeting. That aspect will benefit tune only and heavily modified cars alike. It's in ALPHA now but when it gets through to BETA I'll throw up a post here with more information on it. Until then here is a quote from the other forum explaining a few of the differences.
The isolated control effectively disconnects boost management from the
DME completely. Allowing the JB4 to independently control boost and
throttle. And in doing so we're able to deliver snappier, more linear,
and perfectly predictable boost targeting response across a much wider
range of conditions. We're no longer forced to work around the DME
duty cycle limits and throttle logic. Which means we can implement our
own more performance oriented logic.
The integrated control scheme we've been using since the JB4 was
released is fairly extensively developed and generally works very
well. But with only a few weeks of development the isolated setup is
showing it's potential by pulling ahead in terms of precision,
consistency, and most measurable boost control categories. As a result
we're seeing an improved "driver enjoyment" rating from customers
testing it. So I expect in the coming months iso control will
completely replace the integrated control as far as the G5 board goes.