Engines generally like a good deal of timing under part throttle conditions (high vacuum, highway cruising.) Fixing the timing at a reduced level will result in more fuel consumption and can make the car fee sluggish. Increases in timing with engine RPM, up to a point, also help with efficiency and responsiveness.
As well, retarded timing can increase exhaust gas temps which can stress exhaust valves if severe enough. On the up-side, NOx production falls when timing is retarded though HCs may go up.
I would think that to fix the timing on a 4.6, you will need to have control of the computer.. and if you have control of the computer, you can easily set the timing to be ideal.
Just as trinity said, the ideal timing changes with load and rpm. If you lock the timing to something that performs well at WOT, it will be much more sluggish in normal driving. If you set it up for normal driving, it will rattle like crazy under heavy load. Even at full throttle, the ideal timing changes with rpm by as much as 6-8 degrees. Your emissions will be poor, you'll get worse gas mileage and the car won't be as much fun to drive.
In short, don't do it. You'll make the car run crappily. And again... WHY would you want to do that?
I dont want to do that... The shop who tuned my car locked it at 15* and I cant afford to take it to another shop for a retune... so i just wanted to make sure i wasnt going to **** anything up.