Sorry to be so late to this... I wouldn't worry about efficiency loss one bit, and with some system and process refinement you can get your efficiency very close to fly sparging. I batch sparge, and my efficiency is 80% which is better than many fly-spargers
IMO one big key to this is that I thoroughly stir my mash (with a drill-powered wine degasser wand) as I'm doughing in, and I stir thoroughly again at 30 minutes.
In the end, at the homebrew level (5 - 20 gallons) your efficiency number doesn't matter so much, at most you might need an extra pound or three of base malt... the most important part is that you know what it is, and it is relatively consistent.
Once you dial in your system and calibrate your brewing software (Beersmith, Beer Tools Pro) you can be confident of hitting your numbers on every brew day. IMO predictability and consistency is much more important than worrying about efficiency (assuming, of course, that you're not getting a ridiculously low efficiency like 40%!)
I've said this many times: IMO the single-most-important number to hit on brewday is your pre-boil gravity. At this point in the brewday it is very easy to adjust (add some DME to raise your gravity, or water to reduce). You should already know your boil-off rate for your kettle, so if you hit your pre-boil gravity you will also know your post-boil OG. And knowing your efficiency -- whatever it happens to be -- is key to hitting that pre-boil gravity every time.