Well, those shift characteristics sound about right for a stock trans. My 95 V8 felt like that too and the only thing that helped was the J-mod. I'd do that, change the fluid to Mercon V, new filter and call it a day, as I'd bet that the place that changed the fluid put Mercon III back in it since that's what most books call for aparently.
As far as the cooler goes, you could run it in-line w/ the heat exchanger built into the radiator to help it warm up on those cold NY days. I know that the B&M Supercooler I bought has a bypass in it that allows the fluid to flow straight through w/o being cooled until the fluid reaches a certain temp. All I know is that a cooler will go a long way towards extending the life of your transmission. Besides, I'd rather spend $60 on a cooler now, then over $1k on a trans
Take a look at the tech articles for info on the Jmod. It's not as hard as it sounds as long as you take your time. I know some people don't like opening up the trans. for fear of messing something up, but heck, if I can do it, then you can, lol...
Jay