First off, are the Choate magazine extensions just as good as the Wilson tubes or even the Remington factory tubes?
Second, is there really a noticeable difference in the Sppedfeed IVs and SpeedfeedIV length of pull and is there a thinner butt pad that can be used on the SF IV stock to make the LOP shorter?

The Choate extensions as as good quality-wise as the others, in my opinion.

I think the Remington factory barrel clamp is the best on the market.  The Choate isn't as good, and the Wilson doesn't have one at all and you have to buy one extra.

Some people prefer Wilson's sling swivel system over any other.

Price-wise, the Choate is the "best buy" especially since it comes as a complete kit, and is easier to get than the Remington Factory.

I "THINK" the Speedfeed stocks can be shortened.

The Speedfeed IV-S has a 13" LOP.  There is a "SWAT plate" that you can mount on the Speedfeed stocks that reduces the length by one more inch. However, this plate is exactly that...... a plate.  There is no recoil limiting properties to it.

I had a IV-S on one of my 870s, but recently sold it.  I decided to go back to the conventional stock setup and went with a Hogue overmolded stock and love it. PLus the Hogue only has 12" LOP