I made an oatmeal stout last year and used floor malted maris otter for all of the base malt. I don't know why the LHBS said 20%, but I'd say they don't know what they are talking about. It is a base malt so there is know reason you can't use it for all of the base malt. This is the description on Northern Brewer's site:
This is a special craft malt from Britain's oldest working floor-malting facility. Still made by hand using many of the same methods as they were 200 years ago, Warminster's Maris Otter is a complex and bready malt. Use in any British style and many American styles for unparalleled depth of flavor.
I wouldn't spend the extra money on it if your not going to use it for all your base malt. Do or do not, there is no try!