I've demo'd S10 twice (well.. once the 13 and once the 14.. no real difference to speak of).
I've ridden it in stock form, so no idea how reflash affects it, but yes.. fueling is not very good on the stocker. Definitely stuttering at lower RPMs. Give it a very quick blip and the RPMs never come up.. it does the opposite.. almost feels like it cuts out and then RPMs come back to idle.
Besides that, at 5'10" I found buffeting to be quite bad. I'd have to look into aftermarket screen or come up with a mod for stock one, to ride this bike.
The real deal killer for me though: it just doesn't ride all that great on the road. It's a couch.. soft and cushy. This makes sense for something that can handle offroading, but if 99% of my riding was to be on road, this bike would not be on my list.
Power may be another point which reflash addressed, but in stock form, it felt anything, but strong with quite a bit of unpleasant vibration at certain RPMs.
I test road the new V-strom 1000 (the one with the beak), the same day and pretty much everything about that bike felt better to me.
On the fly adjustable screen with far less buffeting.
Far less vibration.
Significantly wider power band with much better fueling.
Not nearly as couch-like.. very road-worthy, though I imagine it's not nearly as capable offroad.
Of course, all of the above is just my opinion and you may have a completely different perspective on this.
Looking forward to reading how your test ride goes!