What effect will such a thick mash have on efficiency/conversion?
A little late, but just putting some science behind mash thickness and conversion (limit of attenuation) for future reference.
Per Braukaiser's conversion analysis, see graph below, "The results for mash thickness were somewhat surprising. Contrary to common believe no attenuation difference was seen between a thick mash (2.57 l/kg or 1.21 qt/lb) and a thin mash (5 l/kg or 2.37 qt/lb). Home brewing literature suggests that thin mashes lead to more fermentable worts, but technical brewing literature suggests that the mash concentration doesn't have much effect in well modified malts [Narziss, 2005]. Briggs cites data that doesn't show a change in fermentability when the mash thickness is changed [Briggs, 2004]. This was confirmed by these experiments where all the data points were on the same curve that had already been establishe
d in the temperature experiment. "
To summarize, mash thickness is one variable that has little impact on enzymatic conversion. That said, as mentioned by others a thinner mash does enable the mixing to be easier, especially when you get down to 1 qt/lb which is quite thick for a mash. One other benefit of thinner mashes, per Gordon Strong, is that they make it easier to get more consistent temperature throughout the mash. In addition, if you don't heat your mashtun, then loading it with more liquid will reduce the temperature loss over time.
Given this, in my opinion, no need to mash very thick unless you need space in you mashtun for many temperature steps that are being raised via boiling water. I use ~1.5 qts/lb for standard mashes and ~1.85 for decoctions. If doing a step mash, I may start as low as ~1.25 qts/lb to ensure I have enough room in my mashtun for temperature raises and sufficient sparge water available to obtain reasonable efficiency.