How much does mash temperature variance affect attenuation?

I've been dealing, for a while now, with a problem with overattenuation, and I've been unable to get to the bottom of it. My beers seem to fairly consistently finish about 6 or 7 points lower than BeerSmith projects (EG, Beersmith projected 1.018 on a recent SMASH brew, and it seems to have finished up at 1.011).

I just reviewed all my processes, and went back and checked my thermometer again (I could swear I've checked it before, but apparently not?). It seems that the thermometer reads about 2.5 degrees high, so I've actually been mashing about 2.5 degrees lower than I thought I was. I typically mash around 152, and lose 1, maybe 2 degrees in an hour. So that tells me that by the end of my mash rest, I'm sometimes actually just under 148, not just under 150. As a comparison, the previous brew I somewhat intentionally overshot my mash temperatures by two or three degrees, and wound up hitting BeerSmith's projected FG right on the nose. I'm guessing that's not coincidence.

I could see the 2.5 degrees cooler mash temps leading to some level of overattenuation - but would it lead to this much of it, or is there still something else I need to track down in my process?

Are you using Beersmith for adjusting the water volumes? Have you accurately measured deadspace and Loss? Basically if you have more deadspace than what you think, your gravities will come out low if you sparge to get to a specific final volume. i.e. your using more water in the process to get to your final pre-boil volume. You didn't mention if you're hitting your OG so this is just a guess.

Yeah, I didn't mention OG - I'm hitting OG, or very close to it, pretty consistently now. And on those brews where my efficiency was off (took a few tries to get my mill dialed in right) I added DME to adjust OG to what was projected. If anything, I may be a couple points high on OG when the mill's dialed in (I've had BS set for 70%, and seem to be hitting 72-73% pretty consistently now).

the difference between 148 and 152 is pretty significant for fermentability as I understand it. I think you've nailed the problem with your over attenuation. Just shoot for about 2-3 degrees over your desired mash temp and you should be fine.

Also, how much temp are you losing over the total mash time. My Cooler is pretty consistant at 1 degree over a 1hr mash. If it's more for your equip, you may need to adjust your mash in temp so that the mean temp is what you are shooting for.

I was just rereading my copy of How To Brew this morning (the all grain section), and it explains that 149 degrees will lead to the most highly fermentable wort. Palmer's test suggest that the difference in FG between 153 and 149 degree mash is pretty significant. For an 1.050 OG beer he suggests that the FG difference could be 5 to 6 points. I think the mash temp is your issue.