lately Ive been using an air powered sheet metal shear and 3" high speed air powered cutter. Both of which I got from Harbor Freight and have been very happy with them. So far I haven't experienced any warping of the metal from using the high speed cutter but the longest cut ive done so far was no longer than 12-16in.

http://www.harborfreight.com/cpi/ctaf/Displayitem.taf?itemnumber=90114
http://www.harborfreight.com/cpi/ctaf/Displayitem.taf?itemnumber=36568