An alternative view - just work out what actual efficiency you are getting, and add a bit more grain to compensate. It costs pence.
That's the problem with AG kits, they're made up to a recipe with as assumed efficiency, most brewers I know all get different efficiencies due to equipment, method etc. so they'll never all get the same OG advertised by the kit. You can spend a lot of time and effort chasing efficiency, like I did, when all you're saving is a few pence in grain costs. Just buy separate grains rather than kits and size accordingly.
I followed your advice and did a double brew day, added an extra kilo of MO into the mash along with a few handfully of oat husks , batch sparged as quickly as I could, it was the least stressfull day brewing ever, both turned out bang on the OG I should have got from the kit.
I also did another double brew day with an added kilo in each and spent over an hour sparging each, ending up a few points over expexted OG and they both taste great, I know which process I prefer.
I know I have a lot to refine and will over time, but I want to enjoy my brewing, not have it add to lifes stress and anxiety over results.