I would like to add my thoughts on this.
Most manufactures do a "fitted" golf shirt, just viewing the Masters made me think how far these guys had come in terms of fitness and body shape etc, i mean did you see the size of the shoulders on tiger !.
My pal has done exactly what you are proposing, called Levente Golf, and to be fair his stuff is good quality at good prices, but even in XXL i cant get anywhere near it, as cant most of the average golfers.
Looking at the profile of the "AVERAGE CASH RICH" golfer (now i haven't actually done this properly so i am taking a Guess) i would say he is 35+, married, couple of kids, plays on a weekend with his buddies, goes for a beer after a game, and is larger than your athletically fit guys. These are the guys spending there money and these are the guys I would be going after.
Golf is expensive as we know all to well, recently i went on a shopping trip for new gear, clubs etc. I went back this week to get some summer tops and shorts as the weather has turned here in Spain.
We have a huge where house type place up the road selling most brands. You can see my size from the topic with my swing.
From the whole shop i could only find 1 top from Nike that fit me (XXL) and the rest from Asworth (who are known for big sizes) Only one brand with a pair of shorts that fit properly , Cutter and Buck. Yes i could find other pants and shorts but they either needed pulling up round my neck or on my hips.
The problem with this is the shorts were over €80 and 3 tops came to €180 so i got a roasting when i got home. The other thing wrong with it is the Ashworth tops are cotton, and i would like to take advantage of the materials like Dry Fit from Nike, Climacool from Adidas, the Nike top i got is great, its kind of perforated. (i have looked on-line for others but cant find them, its 100% polyester )
So to wrap this up i would go the other way, produce a line that fits us normal to bigger guys COMFORTABLY with hi tech materials and i would by your gear straight away. And dont forget us bigger guys make up most of the market AND ANY BIGGER GUY WILL TELL YOU........................................ WE REMEMBER WHAT BRANDS FIT US PROPERLY !!!