I've fit a bit over 6kg in mine, but the circulation is slower and I seem to get lower efficiency, which negates the point of the extra grain (to some extent - ymmv).
I think around 5kg is the sweet spot, go with that much, and if you want higher gravity, mash in with less water, boil longer, add extract to the boil, or do a double mash.
Also, keep in mind that if you have more than 26L at the start of the boil, the elements don't have enough grunt to boil it as vigourously as usual, so your boil-off volume is lower, degreasing your OG as well.
I had a reasonably disasterous run where I used about 6kg of grain, which lowered efficiency, then I sparged more than usual (to rinse the extra grain), so I started with too much volume, so the boil-off wasn't as high as expected. I couldn't boil longer once I figured this out, because it would have screwed up all my hop additions, so I added a bit of dex and still missed my OG.
Does anyone know what the minimum mash volume is with 5kg of grain?