Why would you assume progressive springs are better off road? Do you want the spring rate to increase as the wheel travels over an obstacle? I'd think the right spring would depend on what kind of off road you're doing, mainly if the wheel is traveling relatively fast or slow.
Progressive rate does seem intuitively better when you consider the difference between unloaded and loaded. At least I know that when I setup my suspension for ride quality and height with a camper and outfit it rides more like a prairie schooner with an empty bed and no topper. But it's a lot better than stock, which was either great empty or zero travel with anything more than 250 lbs.
Pickups rear leaf springs usually are more or less linear and institute progressiveness with overload leafs from the factory. IOW, my Hilux had 3 main, full length leafs that did the majority of the work and a big diving board overload for when there was stuff in the bed. There is a small window of payload where the springs are linear. The OME springs I put on also have 3 main leafs (IIRC) but have 3 or 4 decreasingly shorter leafs that progressively do what the one large overload did in the stock springs (and they have a slightly higher overall max capacity). That makes it ride better through a larger window of weight in the bed but empty my truck still rides rougher than stock because even though they are progressive the stack of 7 leafs is still stiffer than 3.
Point being is linear springs seem to me to ride better daily but progressive springs are higher performance over more cases. The ideal situation it would seem is to have linear springs at the correct rate for your payload with a soft (e.g. progressive) bump stop, but then they would be too stiff with less weight and bottom out with more.
'91 Toyota Pickup
'09 Kawasaki KLR650
'12 Gunnar Rockhound 29
"To live is to suffer, to survive is to find some meaning in the suffering." -- Friedrich Nietzsche