I actually think the grinder caused more heat into the panel and more warping than the welder did. I am biting the bullet and getting a 5 inch grinder this week and also some super coarse discs. As someone commented on Facebook, the bigger the disc and the faster it takes away material, the less heat that is transferred.
i use fibre backed grinding discs for the majority of any grinding work i do. i find they are easier to use and come in a variety of grits as well giving a better finish than a flap disc or normal grinding disc