Should GLP studies carry more weight than non-GLP studies?

All other things being equal, GLP studies would be preferable. However, individual protocols should be compared, before making a final determination.

Yes, However, on a case by case basis a non GLP study conducted with an adequate protocol should be considered fully reliable.

Generally speaking yes, but there are other elements that play a role also. Non-GLP studies cannot be automatically over-written by GLP studies for the same endpoint, a careful weighing of the detailed protocols are needed.