To the original poster, while I do think the OE Toyos are junk, I agree with you that my Mazda5 is the worst fwd car I have had for winter. The problem, as you state, is more in initial starting on any slippery surface, especially when turning (like making a turn onto a new street) or on an incline. Now, let me state, I have been road racing and rallying for more than 15 years so I have lots of car control training and experience. Also, I am a native New Englander who skis more than 30 days a year with many trips in winter conditions. I am not a novice or unskilled winter driver.
Once underway, the 5, with snows, is actually a lot of fun on unplowed roads, very controllable and willing to carry speed and take corners. The issue I see is going from a stop or near stop on slippery surfaces. I believe it is due to Mazda using the same springs from the Mazda3. This affects the rear of our cars more as most, if not all, of the additional weight is in the back due to the increase use of glass. The soft springs cause weight to transfer to the rear when starting from a stop, unweighting the front wheels, and causing a lack of traction. Tires can only hope so much, we are battling weight transfer. I would love to get some stiffer springs that maintained the stock ride height. That would be the ideal solution, in my opinion. I will most likely be selling my Mazda5 before next winter, too bad, it is a lot of fun to drive in most situations.