I installed an EJK EFI tuner a week or so back, and have done about 500km with it so far. It comes pre-configured with a "Stage 1" setup - basically only an increase in the "acceleration" fuelling - but it can easily be tweaked significantly across "cruise", "acceleration" and "full throttle", plus the rev ranges where the shifts in fuelling take place. It also comes with an O2 sensor plug which sets the O2 rate for the factory EFI unit at a constant value, just like the Xieds on Sportsters.
My W800 currently has no changes apart from modified final baffle tubes but even with the standard setup it makes a noticeable difference. Like the Xieds on my Sportster, it makes idle to 2500rpm running much smoother - the bike will also pull cleanly in top from 1500rpm now (no I don't ride like that, testing only) and the extra bit of fuel does make the acceleration a tad quicker and smoother. So far the fuel usage hasn't changed from the previous tank averages over the same sorts of riding conditions including a couple of hundred km of caning it through the Hills. I bought it with the intention of putting a Staintune or BEET full system on at some point, but installed it on the standard bike first out of curiosity. Around $300 shipped from the USA, takes 10 minutes to install and can be reset to factory specs in seconds or even tuned while riding if you mount the controller panel somewhere accessible.