Personally, I wouldn't want to take the time out to leave the park to travel to Kona for lunch, and then return to MK again. You are probably looking at 1.5 to 2 hours total, between the travel time (exiting and re-entering the park, then catching a monorail or boat each way), checking in and being seated, waiting for your food, eating, etc.. But that is just my opinion, based on my family's priorities when vacationing at WDW.
We find it a more enjoyable option to schedule an early breakfast at Kona, prior to park opening--and then, since we had a filling breakfast, we just grab a lite CS lunch inside the park. Columbia Harbor House is one of our MK faves, along with the Main Street Bakery. A lot of people like Cosmic Rays, also--though we found the food there to be too greasy for our taste. If you want to sit down for a TS lunch inside MK... Liberty Tree Tavern has good food served in a relatively relaxing atmosphere, while Crystal Palace puts on a nice buffet spread with Character interaction (Pooh & friends).
If you are staying at the Poly, it can be worthwhile to try lunch at Kona while taking a mid-day break from the parks... or we will sometimes stop over for lunch there when park hopping, from an MK morning to Epcot for the evening. Kona is also a good potential lunc spot for on a day where you are touring the various resorts, as opposed to spending all day at the parks. But otherwise, it is probably a more efficient use of your time to eat lunch inside of the park you are visiting.
Yes, in most cases you would be very wise to make an ADR for Kona. They sometimes do take walk-ins during the slower times... but you may have a long wait for a table without an ADR (and risk being turned away entirely). It would not be fun to take time away from the parks to travel to the Poly, only to find they can not seat you because you hadn't made an ADR.