Hiya,

I just got my P20 a couple of months back and recently did a 50km ride with a rack and 2 small panniers. It rides very nicely with weight loading as i've also done steep climbs with my 5-yr old son on board. So the gear ratios are great both for speed and climbs with bags/son. My wife rides a P9 so we have swapped bikes midway and noticed the P20 really being more responsive, quicker and easier. The Kojaks play a role in that as the P9 are with Marathons. Not sure yet how the Kojaks will hold up on rougher roads, but they sure feel more jarring on the bones than the Marathons.

You should also note that some P20s like mine have rear rack braze-ons (on the seat-stays) that are asymmetrically located. Some folks here don't have this challenge, which just means that your choice of rear rack will need a modular fitting system that allows for an asymmetrical fit. I use a Pletscher Athlete 4B for 20in foldies and it works great.
Another thing is the Sram X7 rear derailleur is nerve-wrackingly close to the Kojak on the largest cog, perhaps about 1/2 inch, so fitting on Marathons will reduce the gap even more!