Thanks for the pict's YG

I'd recommend the stock hole.  You may prefer not to be in here for another 10k miles but I think you're going to get accelerated wear.  With the stock hole it may go another 10k before you have an issue.

I'm thinking that stock extention brand new will be somewhere between 8 and 10mm out.  So at 15mm, you have used a little more than half your life on the chain.  And this is with the higher spring tension at it's most compressed state.  I'd expect that your plunger will need a reset at 15k miles.

But, resetting it now, you may need to replace the guides and chain at 18k.

Your call, but I'd put it in the stock hole and check it in 5k.  Or wait for it to get noisey.  Remember, you don't have to worry about blowing up on you.  It will bottom out, the chain will get loose and noisey.  Even bottomed out it will last a long time before the chain will be loose enough to skip a tooth if you can stand the noise that long.  

I guess you could find out. You got any Giunnea Pig blood in ya?

Brand new install, verslavy with old style spring and new rear 'tensioner' guide, my plunger extended 5mm.
1k later, it was 6mm.
2k later, it was 6.7mm
further reports to follow... toot squeak, or whatever the he77 hamsters say  

My advice, 1st hole until you need it to give you the most out of you system.

After doing as good as I could with a spring scale, marks on the tensioner, and shaky hands, here are the results: (eye on scale and measure right before it "clicked" and landed at the mm mark indicated)

0 mm = 7 1/4 pounds
5 mm = 6 1/4 pounds
10 mm = 4 3/4 pounds
15 mm = 3 1/4 pounds

It looks like I'll be opening it up to put it back in the 1st hole again.  At least I know what to expect now, and the gasket should still be fairly flexible and easy to save.