That cut will give you less volume, flimsy handles, and less boil-over protection. However, it will allow easier pouring (if that's what you plan on doing) and a larger access "hole."
If you are already using a turkey fryer pot, you can cut the hole in the top so that the lid from that pot will fit.
2nd. I traced the lid of the 22 qt pot i used to do extract brews and i use that as the lid. i like having a glass lid as i can peer in to see how everything is going.
__________________ Jaded Spirits Brewery
Primary: Dennis Hoppier III
Secondary: Empty
Bottles: Triple Berry Apple Cider, Cote Apple Cider, Wild Blackberry Apple Cider, Dim Wit II, Mighty Porter
Kegs: 4Cs IPA - The Diamond Standard, Innis & Gunn Rum Cask Clone.
On Deck: The "Three Stouts", Wild Blackberry Mead, Irish Red
I jumped right in. After watching all the videos and seeing Yuri's awesome plasma cutter (which I don't have) and Bobby's rig, said F-it and took my rotozip with the angle grinder attachment on it and started cutting. I didn't mark it or anything. I got perfect circles and it didn't take forever like some have posted it took like 5 minutes a keg, if that. It was very easy just don't over think it. Not saying its for everyone but sometime you gotta jump right in and see what you can do . I could post some pictures of them if you want.