Anything you put on TOP of your seat will only sit you higher, while at the same time, take the same shape of your seat. Motorcycle seat comfort isn't necessarily a function of the foam, so much as it's a function of the seat shape and design. You're right, the stock seat on the 900LT will shove you forward into the nose of the seat and cause the infamous "monkey butt."
I tried a Mustang seat and it didn't make it for me. I think it just put me in the wrong posture. A Russell Daylong saddle was the cure and worked perfectly for me as it is a true custom-made seat. A Russell is expensive, but definitely worth the money spent, particularly if you tend to ride long distances.
While the type of foam used in a seat is important, it has to be coupled with the proper shape. The foam alone doesn't do the job properly. Seat pads may seem to be more comfortable (and, to an extent they are) but, IMO, they create a feeling of being somewhat disconnected from the motorcycle. A good custom seat is fashioned similar to those famous tractor seats and distribute your weight evenly and properly on the seat, and a true custom, such as the Russell is not only made properly, but is made to your individual specifications, hence when you order one or ride in and have them make it for you right there, they want your weight, height, inseam measurement, any specific physical issues that need addressed, and photos of you sitting on the bike.
Sorry for the long reply, but if you're like me and have back issues, as so many of us do, then a proper seat is paramount to motorcycling comfort. I've had several Russell seats and have never been disappointed.
"Don't sweat the small stuff---it's all small stuff"
'09 650 VStrom