I am about to apply my very first "dose" of SBM. My question is, if I have a thick thatch layer(which I am looking to eliminate), will the SBM be able to penetrate the thatch to begin to "work" the soil and break down the thatch?
Microbes will find the SBM whether or not it penetrates the thatch. Be patient. It will take a few weeks to see any effects of your first dose. A good rain might help. You will not totally eliminate thatch, but an organically maintained lawn will eventually have a healthy, acceptable thatch layer. A light application of compost would help get the microherd munching on the thatch.
If you're real concerned about the thatch, as I was with mine, I would highly recommend spraying some compost tea. I think it really helped my thatch issue. I put down a dose of it around the beginning of May and I've seen a HUGE decrease in thatch since then. The SBM will help, too. If I were you, I'd put down both -- that's what I did. Keep in mind that I also raked the REALLY bad spots, but I think you'll see some good results with SBM and compost tea.
Compost tea will be a LOT easier to spread on your lawn than compost -- especially with the size lawn you have. Compost would help more, but putting a layer of compost on a 1 acre lot might just kill you...
Compost is great for moisture retention too. I applied some in March. The compost is still moist several days after rain in very hot weather.
SBM is great stuff. I put out another round two weeks ago and the results are fantastic.