Hi Dustin,
I'm strictly an index futures trader, but I find pairs quite interesting. So, just some random thoughts about Pairs trading:
One danger is liquidity. In using a longer timeframe, you are not basing a position on a single tick of one of the pairs during the day. Arbs will be watching the more popular and larger issues, and that errant tick will be brought back in line very quickly. I think that there's a certain minimum timeframe that someone can successfully trade a pair, and that's driven by the issues you are trading, the trade routing (ECN vs. Specialist).
I like what you are doing by evaluating the value of a move back into the "normal" zone.
I would want to make sure to backtest the action of the pair over a very long historical period. When choosing 1.5 std dev. over a 5 day time horizon, there is still a considerable possibility that this pair will continue moving outside of those bands. I'm assuming that you are using 2 minute bars, from the chart lower on the page.
What I really like doing is taking something like a three or four year intraday history of something like 5 minute bars, and find all of the levels and their associated probabilities. Like, in your Excel sheet, let's say you have a 3.5 Std Dev close (or alternatively, say 5.03%). With a large amount of historical testing, and an understanding that this is say an 88% trade with a very small adverse excursion, I'd be confident on really leveraging that opportunity (much like being dealt an 11 in blackjack with a very favorable card count....). Other values, I'd size my position accordingly.
The advantage to a pairs strategy is that you can pull one of the legs off the position if you want to work with your position timing. So, you can do a longer term trade, with some short term directional stuff as part of it.
For example, say you sell the MXIM-LLTC spread, and it keeps moving against you. You can take off the short MXIM intraday and try to time its turn, all the while holding the LLTC long (presumably it's increasing somewhat with the general trend of the market--I'm assuming it's up for this case). This adds risk and commission/slippage, but can be an additional tool for finessing the trade.