Re: the purpose of flat vs. progressive, virtually all street-focused motorcycles come equipped to carry a passenger. As a result, the rear suspension needs to be able to support a lot of weight placed high and over the back end to avoid suspension bottoming and generally dangerous behavior. Without a progressive link or spring, you'd have to have a spring so stiff it would beat a single rider to death and lack compliance over the usual road imperfections we all encounter every day.
So manufacturers connect their swingarms to their rear shocks via linkage that provides a progressive ratio. What that means is that in the first part of the suspension travel, 10mm of vertical wheel travel might only compress the shock 3mm (3.33:1 leverage ratio), while in the latter part of the suspension travel when you're approaching bottoming, 10mm of vertical wheel travel would compress the shock more, say for example 7mm (1.43:1 leverage ratio). So it really just changes the leverage ratio so that the "effective" spring rate gets stiffer as the rear end compresses, and softer as it extends. Offroad, the same thing is done to get compliance and traction over smaller bumps with sufficient bottoming resistance over big jumps and impacts, and some of that applies to the street as well.
Some progression is appropriate anywhere, and the more bumpy the surface, the more progression you want up to a point. Too much and the rear end of the bike will wallow, squat and then pack up when it hits the stiff part of the progression curve. Even Ducati's F, "flat" link position is slightly progressive; Roadracingworld published it as having a 2.14% rate of progression at 100mm of wheel travel vs the P position's 14.15% at the same travel. And even the P position is less progressive than the link on the 1198 was, going by Ducati's published graphs. Both F and P positions have the same 2:1 overall motion ratio, so they return the same total wheel travel for the same total shock travel; it's just that with the link in P, the effective spring rate is relatively lower (softer) at first than in F, and then gets higher (stiffer) towards the end.
With the stock spring and riding solo, using P will make the ride smoother over small bumps than in F, but note that it WILL get stiffer once you hit somewhere around half to two-thirds travel. This is so by definition, given the same overall motion ratio; it HAS to catch up. Racetracks, even those less endowed with TLC than new and spendy ones like Yaz, are a ton smoother than public roads. So you don't need to worry so much about bump compliance, plus you're not carrying a passenger, therefore a small amount of progression is appropriate. On the track you're more concerned about chassis pitch control and stability under severe braking and acceleration, which a flatter curve is better for, while on the road you're dealing with a much more variable set of circumstances, thus the need for more adaptability (progression).
I'd say if you're blessed with roads that are particularly smooth and you ride solo all the time, then using the F position and getting the right spring for your weight is likely the best option. However, if your roads are gnarly and/or you tend to like company on back, then P is the way to go, again with a spring change if the stocker's not ideal for your weight. As light and mass-centralized as the Pani is, it's just naturally going to very sensitive to setup for the rider since the rider's a larger percentage of the total package, with the resulting changes in CG, MoI, etc.
Hope all that's of use. Bottom line, budget for springs at least, and tweak away. The Pani is just a motorcycle, with a certain wheelbase, mass, CG, MoI, and suspension. It is tuneable just like all the others, it's just new so the knowledge base is sparse.
