OK, I looked at this at lunch and feel the urge to throw my humble opinion in.
First, I am a Ft. Wayne Member and although where I will be and what I will be doing in the future is very much in the air, I do still care about my club. Although I have run SMS in the past, my SMS vehicle is currently sitting at the house of someone whose drive looks like a Subaru lot with a Forester motor dropped into it that has a knock and I no longer own it, so I have no personal interest in this issue.
(SM) X ("Street Tire Multiplier") vs. (SM + STU)/2: I vote for the street tire multiplier. This was discussed when Chris first proposed this class, but for some reason, the somewhat arbitrary STU-SM average was agreed upon. I agree with Rick's reasoning and if anyone wants the basis of his 97 to 97.5% number, I am sure he can provide it. I do know (and I shared this back when this first came up) that there are some regions (Detroit in the eraly 2000's that I personally know of) that do allow street tires in Stock Classes to use a multipler to even things up for those who do not want to have the hassle of changing tires/trailering.
Does anyone care about SMS PAX: Well, as has been said, the original intent was a catch-all class for those who had made mods that put them out of ST.., SP... etc. and did not want to run R-compound. So, by definition (at least regionaly), this was a class for those who did not prep their cars purposefully to compete at a national level. For the most part, until Chu...er, uh...Mr. Ucter has shown this recent interest, their really has not been anyone that intense in the class. This is not to say that those who run SMS are not good drivers, nor drivers interested in getting better, nor that their cars are not fast (some are quite loud and shred tires on command), nor that their owners are not improving themselves or their cars as time goes on. It is just that these are people who want to run what makes them happy and not worry about running out to buy a Shelby Mustang for FS or a Mini S for GS just so they can be in a "spec" class that runs Nationally. Having said that, no one wants to feel that their class is not PAXed properly. Of course, the traditional way Rick goes about his business for developing PAX is based on actual results of over 200 National, Divisional and Regional events cannot be followed in this case because it is not a "National" class, hence this interesting discussion.
National PAX vs. FWR Events: I would be curious how Rick goes about his number crunching, but the Chicago region site notes that he does use data from National and Divisional, as well as multiple regions events. I do not know if he uses all data or only those who throphy. But, let's face it, we tend to have smaller courses with loose surfaces. This is not the same as larger sites that may have concrete (Peru or BAX in Toledo) or decent asphault (read - potholes do not spring up or enlarge as the event goes on). I will pick on my self for a moment. In 2008, I drove a 2001 Subaru Impreza 2.5RS in STX. An Impreza in STX, why that's just the ticket, you say. Well, that would be true if I had a WRX, which has the 220hp 2.0 liter turbo engine, vs. my 165hp 2.5 liter NA engine. I was in STX because of a cat rule that I violated because my aftermarket header put one cat in instead of two (and before that, it was that you could not have an aftermarket cat). So, some might say that I was not in the best vehicle for the class and that I was PAX challenged. I happened to do OK that year, not because I have the raw tallent to be the next Schumacher/Hamilton/Vettel/Loeb/Etc nor because I have several decades dodging cones (which I don't). It probably had to do with the surfaces we run (AWD helps in low grip situations, just ask Warren Leveque, he reminds me whenever he sees me) and the tighter courses that do not favor horsepower. Also, having the instant and smoother throttle response of NA to help balance the car at the edge (as opposed to turbo kicking in) probably helped as well. I have gone up against the turbo cars and at larger courses, got my behind kicked. But, I did not buy my car to be optimal in my class (would have been STS/ST if I had corrected the cat issue) and I did mods knowingly that put it into a slightly higher class just because that is what I wanted for the car (also my daily driver while it lived). My point is this, the PAX is based on data other than our own and a regional class PAX probably does not have to be accurate to 0.7% (the difference between the two methods above; roughly equal to 0.280 sec. on a 40 sec. course).
Please keep it simple. Go with the Street Tire Multiplier of 97.5% of SM which would be 0.855.
My $0.020314159 worth....