Sinew just won't improve the cast of a longer bow, the sinew just does not get stretched enough to start working. The working sinew causes the belly to take more compression, in effect multiplying the amount of bend the limb is actually subjected to.
I don't think sinewing is as exciting as I used to think it was. I used to sinew back everything, short bows, recurves, quivers, arm guards.... I guess I got it out of my system. So says the guy that is going to whip out a series of 5 sinewed horsebows this summer. But I am doing it as part of a seminar I am doing at the Rendezvous at Fort Union this June. It's messy enough in a garage where I can have buckets of warm soapy water at hand, electric burners to warm glue, benches/clamps/modern electricity/etc. But no, I gotta do do it in da dirt in front of a bunch of touristas!