Improved Stalwart uses Combat Expertise to increase DR rather than AC, doing so at 2 points of DR at a cost of -1 attack bonus for every +4 BAB and does so with a maximum benefit of 10/-.
So here's my question: as a full martial, at 16th-19th level you're suffering a -5 attack penalty to gain DR 10/-, but at level 20 that penalty suddenly becomes -6 with no additional benefit gained? That seems inappropriate to me to say the least...
Then house rule it. Either the penalty stays at -5 when you reach 20th level for this feat with DR 10/-, or the penalty increases to -6 with DR 12/-.
I agree. It should scale like (for example) Power Attack; for every +1 to AC you receive from its list, you can convert that AC to an equal amount of DR.
I do concur that it's a little silly for the extra penalty to be added for no benefit, making the feat more favored towards those who utilize Fighting Defensively/Total Defense options, and makes the Crane Style line of feats that much more valuable.