Done around 400 miles now with my Powertronic fuel controller & DNA filter fitted, so here are my impressions.
Certainly since I started riding in the mid – eighties, 250cc and 500/600cc have tended to be the popular engine sizes for pure road going singles – at 373cc the Duke is pretty much halfway between these two classes.
It just so happens I’ve previously owned an example of a machine from each of the afore mentioned classes – Honda’s CBX250RS & Yamaha’s SRX600………
In stock form, the 390 reminds me of the Honda which also had a very smooth and free revving engine with a 10k+ redline. The 390 adds some low down torque, another 12hp and a harder performance edge, but stock it definitely still feels like a smallish engine – if fairly highly tuned.
An impression reinforced by the slightly wimpy low rev exhaust note.
To put it another way, in standard form the 390 felt like a kind of cross between a 350LC and the CBX250RS – so pretty great !
With the Powertronic & DNA fitted, above 7000rpm or when pinning the throttle hard it doesn’t feel particularly different to before BUT most of the time that I’m below 7000rpm e.g. while wafting along on a fairly steady throttle at 50 to 60mph in 6th, I am NOW much more reminded of the relaxed, torquey feel of my SRX600 (but without the vibes and still with that KTM hard & fast revving nature fully intact when you crack open the throttle).
So with the Powertronic fitted it feels more like a cross between a 350LC and a SRX600 which is just FANTASTIC!!!!
This transformation in the bikes character is quite simply as awesome as it was pretty unexpected (I expected better but not this much better !!) and is certainly aided by the much more purposeful sounding induction roar and exhaust. Winding on the throttle at about 5000rpm now results in noises from the airbox (and assisted by the more purposeful exhaust bark) that seem almost Ducatiesque 
It is just so much more satisfying (and so much easier) to ride at lower speeds now (sub – 60mph). The really fierce engine braking that risked scrubbing off speed too fast entering a bend has gone – I’m a fan of engine braking but you can have too much of a good thing. It has lost that general over- sensitivity to the throttle and is also much less fussy about which gear it is in, I find 4th in particular a lot more useable now at sub 40mph speeds than I did before.
The fueling still isn’t absolutely perfect, but it is a vast improvement and simply no longer an issue.
A 140 mile mini tour around the North Yorkshire Moors was achieved on a single tank and reflected an average mpg of 64. So fuel economy appears pretty unaffected.
I remember years ago british motorcycle journalist Rupert Paul writing about jap market 400cc bikes and saying that rather than being restricted by some artificial capacity class, a motorcycle engine should simply be as big as it needs to be “to feel good”. I’m not convinced that in stock form the 390 satisfies Mr. Paul’s criterion. Yeah it’s good, very good even, but I think most people would think it’s only very good for a 400 single.
With the Powertronic (& DNA) It’s just a very good engine period (as some forum members might say) – With a fuel controller it is big enough to feel good. So good in fact that it is probably just as well for KTM the 390 isn’t like this straight from the factory as it would surely harm sales of the much dearer 690.
I’ve also finally added some EBC HH pads that have provided a similar level of transformation to the front brakes. With these latest modifications, the bike feels altogether a more grown up, more sophisticated, more expensive, more special machine. Only the suspension quality lets it down now.
My brain is now convinced I am on a different, physically larger and larger engined machine – maybe some special limited edition 500cc single cylinder racer (like a manx norton !!). Yet when I get to a bend there is still that same fantastic agility from a 36kg engine in a 125 chassis !
Whichever flavour fuel controller you go for I am sure you really won’t be disappointed, as others have said it has to be the single best mod you can do – highly recommended !