Shift kit: The 4T60-E transmission is one of the best ones GM makes. It's durable, light, and has allot of electronic components that make shifting clean and precise. However GM takes into account that most people don't want firm shifts. To overcome this TransGo makes a shift kit that improves over 70 components including shifting at full throttle. If you call TransGo at (626) 443-4953 they will give you the name of the nearest dealer in your area to buy one. The part number is SK4T60E and price should be around $67.00. Installation should be done by a qualified transmission shop.
Transmission Cooler: It's always a good idea to keep your transmission as cool as possible. From the factory GM has the transmission lines go through the radiator to cool them down. GM has always offered external transmission coolers as a option. And some of the models came with them standard. If you do not have one there is a solution. Go down to your local GM dealer and have them look one up for your car. The factory transmission coolers will fit right in front of the radiator and there are already holes in the sheetmetal drilled for the bolts. Price is $30.00.