I have a front loader. So could I soak the batting in a tub of hot water then spin it in the washer and finally dry it in the dryer?

My front loader has a rinse and spin cycle.

I am actually getting ready to do the same thing. I broke down and bought some poly batting at the LQS. It is Quilter's Dream and feels amazing. I tried shrinking Warm and Natural. It shrunk a half inch per yard with a single washing.

My current plan is to use the poly and the prewashed W&N on two matching flannel quilts I made. The flannel was prewashed. I will then wash and dry them both and see how they turn out it comparison and make my final decision on what to use on my jacket based upon that.

You definitely want to preshrink before making a clothing item. If you are doing something that is very dark, I would be hesitant to use a poly unless it has a scrim to prevent migrating fibers. Alternatively, use a black poly batting.

I just soak in hot water and then put into dryer. In the summer I hang it out to dry for a while so it doesn't take as long in the dryer and is more gentle of course than the spinning.

i do not pre-wash batting any time-for any project! and i've made lots of jackets, totes, home dec items-
i purchase the batt that is best suited for what i am making- i can simply not even imagine the hassles involved in pre-washing a batt...that just seems---crazy to me- there are so many wonderful batts on the market- with different properties/fiber contents/ shrinkage rates/ quilting requirements- some are specifically made for clothing items- (they say on the package-good choice for clothing items)