Yep, I too am thinking about the air ride seat for my 6 month old L5460HSTC. I have put nearly 100 hours on it mostly mowing with some grapple work mixed in. The Kubota GL60 series tractors are a big improvement over the prior models.
Kubota M8540HDC, L5740HSTC, BX2200, BX2660, John Deere 425&1025R, Case, Massey Ferguson, Ford
I retrofitted a Grammar air ride seat, model MSG 93 - 721 on my L5740 with the help of 4shorts, it wasn't a factory option on the L40 like it is on the L60, and it's a really big improvement.
I bought my L40 to avoid the L60, but agree there are some improvements on the L60. Like in a lot of things though, what are considered improvements are just seen as changes by others.