I find myself running my ARs with the collapsible stocks positioned almost exclusively in the second notch from fully closed.

This stock position measures 9.0" in length from the receiver to the end of the buttplate.

This works fine, but I'm not a fan of the raised ridge at the end of a collasible's plastic assembly.  It irritates my cheek.

I've looked into short fixed stocks like the ACE shorty, the Sully stock, and the BM stubby stock, unfortunately they're all under 9.0".

Any opinions on using either KAC's or DPMS's stock extension spacers?  The ones that fit between the stock and the receiver.  With the combination of one of these spacers and a short stock I feel I can get to my magical 9.0" requirement and improve my cheek weld.

Anyone have experience with this setup?  I'd like to know if it would be as solid as an A2 buttstock and if it would work as I've decribed.

Thanks,

Its expensive but a Magpul stock fixes your issue.

I thought trying your setup for a while...then the ban ended, and I have a Magpul.  I'd be interested to know how it would work and I'm sure it will.  Certainly is cheaper than a Magpul.

Just an option.
I have the same issue with the rib on the 6 position as far as cheek weld.
You can carefully grind the rib down with a stationary belt or disc sander and then hand finish(blend and contour to your liking) with 600 wet and dry sandpaper and OOO steel wool.
You can't tell it didn't come that way.
I replaced all my 6 pos stocks with fixed, except for a light wt. carbine that this fix worked great on.
As far as to making the stock weaker, maybe, but I don't  try to break down doors with it.

I aint no expert.  But in the this Sully stock review from 2004, it appears that the Sully comes with both thin and thick buttplates.  Looks to me like the thicker version would address your need.  

The current Defensive-Edge link shows the stock in it's current config, which looks like just the thinner buttplate.  You might want to give them a call to see if they still offer both buttplate options.

I have the Sully stock. It comes with both thin and thick end plates. With the thick end plate  it is just over 9 inches.  I think the Sully stock is a good alternative if you dont want an adjustable stock.