Chlanny; Are you saying the FACE of the studs are going to be 6" away from the wall which would make the rear of the stud approx 2 1/2" from the concrete wall? I would hope that it 's not 6" from the REAR of the stud, because that can cut down quite a bit on the overall room size. Anyways, If you are using R 20 insulation, that is good when installed snuggly between the studs, and that alone should be quite efficient . Any insulation placed behind the studs (imo) would be "overkill" and just a waste of time and money.
Normally, unless the concrete wall is drastically not straight, a gap of approx 1" is all that is needed.
Ps; depending on the part of Canada you live in, don't forget to install the vapor barrier in the right position.Myself, I live in central Ontario,Canada, and my basement walls have the vapor barrier on the heated side of the wall, just next to the drywall.