I just checked the photo paper that I have, and it is 45lb (170g/m2). This is the paper I used for the Wannabe. After cold laminating the thickness is good for shuffling and dealing, Maybe a bit on the light side, but I like using this paper since the print out is very sharp. Now for the Tarot of the Golden Desert, I mounded this photo paper onto 110lb card stock and then laminated. The reasoning for this was the hexigonal shape called for a much sturdier card for dealing.
I use cold laminate because you can get a much thinner laminate that way.. Hot laminate was just to thick when you go to shuffle and deal, and every once in awhile I find hot laminating to peel off.
Now I've found that this kind of project, where everyone is printing out their own cards, it's going to be difficult to shuffle in the normal way, anyway, different card/paper stock, and slight differences in card size, etc.
It's hard for me to judge if the photo paper you have (270gsm) is thick enough, without having it in my grubby little hands
... but with the smaller size of ATC cards, the cards don't really have to be as thick/sturdy as say the standard RW size...
I know that in the majors only we decided to not laminate. I don't think there has been any decision either way in this one. Personally, I like to laminate, It gives more protection to the card. Especially if you plan on really using the deck for readings, and not a collector item.
The main thing is just that the paper is thick enough to be able to be dealt out (how ever way it is shuffled) and can take some abuse (that's where the laminating is good for).
Sorry if a ramble a bit... Sometimes I find it hard to put down into words what I mean to say...
BB
DraagonStorm