It was recently recommended to me that I should modify my transmission with an additional cooling system and add a high performance air filter. I was told I would get more horse power and that it would add life to my Freestyle. Is this true? And where should I shop for these parts?
Yes what you are asking is true. Both recommendations will benefit the transmission and give a little boost. Just a little not much.
If you are planning to tow a trailer or boat then the transmission cooler would give a longer life to the transmission by keeping the temperatures inside cooler then the normal operating temperature. The same thing applies if you plan on racing the car. Heat is the number one killer of transmissions.
The air filter you mentioned is the easiest way to get a few more horsepower without spending a lot of money. It won't make it a race car but will help a little by letting the engine breath a little easier.
These two items can be bought at most auto supply stores or speed shops and can also be bought at many online sites.