First, you shouldn't assume a particular feature or attribute of a product will end up being classified as a Performer feature. It could be that enough time has passed and enough standardisation has occurred that the requirements for that particular feature has stabilised at some plateau level, and the customer satisfaction response now follows the Basic Expectation curve.
For example, there was a time when the number of pixels a digital camera captures was a Performer feature - the more the better, because there were many cheap digital cameras that were puny in their specs. Today though, for consumers, this attribute is closer to being a basic expectation - takes enough that you can't see the grain, and not so much it takes ages to download one image. Whatever that means. Sure, there are digital cameras with best in class pixel resolutions, but there's still a very big market for "good enough" pixel resolution. Using the Kano Model will help establish if your "good enough" camera can be improved and enhanced into a Extreme Plus Pro™ version, or whether you should focus your attention onto other factors.
If you believe a given feature will result in a classification of being a Performer feature, then you should be expecting the function form of the question to result in "I like it that way" (and not "I expect it that way" or lesser alternatives), and the dysfunctional form to have "I dislike it that way" (and not "I can tolerate it that way" etc responses).
One possible problem with your suggested phrasing is that you've expressed both the functional and dysfunctional forms as specific levels: "poor fuel consumption" and "best-in-class fuel consumption". The difficulty here is that these phrasings may not be equally weighted opposites in the customers mind.
Given all that, your best course of action might be to phrase the questions as follows:
- How would you feel if [the feature] was good enough?
- How would you feel if [the feature] was not good enough?
Note how (a) the description falls into a vague middle ground, and (b) dysfunctional form is literally the negated functional form (i.e. opposite).
With (a), you won't have a big problem with misleading false-negatives where a customer doesn't choose "I like it that way" because they happen to be thinking "Oh, if it's the bee's knees it might be too expensive" (i.e. they're second-guessing knock-on effects onto other attributes).
With (b), you side-step any complications arising from customers who think the opposite of "best-in-class [attribute]" is "'effing woeful [attribute]", with your phrasing of "poor [attribute]" being interpreted more softly as "kinda sub-par". If they read "poor" in that manner, they're more likely to answer "I tolerate it that way" instead of "I dislike it that way" (which you should be expecting if the feature is in fact the Performer class of features).