Could it possibly make much difference which metal 13 by 9-inch pan you use? Several dozen rounds of baking later, the answer was an unequivocal yes.
Cake Baking Times By Pan Size
Pan Size and Volume > Start Cooking
Make cakes worthy of celebrating with this Wilton Bake it Better 9 x 13 in. Non-Stick Oblong Baking Pan. Great for sheet cakes, brownies, cookie bars,
Wilton Bake it Better Steel Non-Stick Oblong Baking Pan, 9 x 13-inch
The Best 13 by 9-Inch Baking Pans/Dishes
ATK Product Reviews, Kitchen Equipment Reviews
Range Kleen 9 x 13 Inches Non-Stick Bake and Roast Pan, Grey: Home & Kitchen
The Best 13-by-9 Casserole Dish
Good Cook Oblong Cake Pan 13 x 9 Inch 1CT (Pack of 6)
The Best 13-by-9 Casserole Dish