Fun with numbers: Dunn with runners on base

I've long held the opinion that Dunn's low numbers with RISP have nothing to do with Dunn's ability, and are in fact the result of pitchers refusing to throw strikes with men on base.  I've utilized this as a defense of Dunn many times, but recently realized that I had no statistical basis for this.  I often get on people for providing opinions with no backup, so not living up to my own standard in this case troubled me greatly.  For those who have not read my full opinion, here's a quick overview:

Dunn typically bats 5th, near the end of the run producers, with inferior hitters behind him.  Dunn is extremely dangerous with runners on base because mistakes cost multiple runs; however, he can't go outside the zone for hits.  Therefore, most pitchers seeing Dunn at the plate with runners on base and a guy like Gonzalez on deck would rather throw Dunn a bunch of borderline crap, hope they get a call, and deal with Sea Bass if they walk Dunn.  This will result in a significant drop in BA (because Dunn can't hit borderline pitches), a significant increase in the number of walks (because the pitcher doesn't always get the call), and an increase in the number of strikeouts (because sometimes they do).

Pitchers are most likely to take this approach with first base open; the lead runner doesn't advance, and you get to face a much worse batter.  Therefore, I broke down Dunn's numbers with first base open vs. first base occupied.  Recall that I believe Dunn to be the exact same hitter in all situations, and the only thing that changes is a pitcher's approach to him.  If I'm right, here's what we should see:

With first base occupied: BA/OBP/SLG and AB/HR very similar to career numbers

With first base open: Drop in BA, increase in OBP, similar isoP (SLG-BA; tells you what he's doing with mistake pitches in those situations), drop in AB/HR (because he's swinging less)

Dunn's career numbers: .248BA/.378OBP/.518SLG, 14.1AB/HR

Dunn with 1B Occupied: .258BA/.388OBP/.529SLG, 13.7AB/HR

Dunn with 1B Open: .204BA/.449OBP/.454SLG, 15.07AB/HR

Here's my question - Which is more likely?  Dunn completely changes his hitting approach with 1B open in a manner that leads to 20% less hits and more than twice as many walks?  Or pitchers change their approach to Dunn with 1B Open, giving him nothing to hit and content to put Dunn on and deal with the inferior hitter batting 6th?