I chucked one in a coles bag. The bunnings bag is much bigger and I didn't have enough potting mix for that one.
It was more of a winter thing, to see if it could escape being over watered due to rain better than a bucket, as it could breathe/evaporate water better.
Tests are inconclusive as it was in baileys premium potting mix, and that's been crap for me. Last year I started off with debco organic for 20 or so plants, putting them in 9L buckets. I ran out of money and baileys was on special for $6 so 30 or so plants potted in that in the end. Most of those plants died in the end.
Even for my first few new seedlings this year, I used left over baileys, and they have been the slowest growing plants I ever had...
Definitely have to track down a decent garden/soil place for bulk organic soil before I repot next month from 330ml and 750ml containers into the 9L buckets again. Can't afford the bagged debco organic or amgrow organix prices anymore since I'm not working right now.
Though if you are worried about summer heat, i don't see the bags being better than buckets, the roots will have to be hotter wouldn't they? I can get away with either as I put my plants under trees, so they only get morning sun anyway.
Doubling up the buckets (or wrapping them with maybe newspaper?) could be an idea. Proper pots aren't cheap, like $6 for a decent 30cm dia one, and 25cm buckets are 72c each, so two of them in only $1.44. Cheaper pots aren't going to be any thicker than the buckets.
The buckets become brittle, last year I bought red and green, the red ones faded and became more brittle than the green for sure. Only problem I had with 2 green ones was cracking at the lip when I picked it up at the rim. The other 20 or so green ones are still perfectly fine handling the same way, but as I said, mine spend half the day in the shade, also I got a bunch or proper pots that cracked after 2 years or so as well, that's why nurseries and wholesalers give them away, cheaper plastic proper pots aren't made to last either (anything under $5-6).
BTW only tried a reusable shopping bag because of this post by cayennemist http://thehotpepper.com/topic/33003-bigger-pots/page__hl__bags#entry670559