A "cheapie Mall Wort" shell is made by one of the major manufacturers and has decent internal components so it WILL NOT shoot a wider pattern than a equiv AA Win load by any noticable amount. A big myth that they are of poor quality, IMO.
You didn't say what the gun was that you were discussing, but you did say you wouldn't open the chokes, so it must be either a vintage gun or a valuable gun as is. If it's a vintage gun, the Fiocci spreaders are all of relatively high velocity and likely high pressures. Give that some consideration before you use them in a vintage gun.
Handloading may be your best bet if low pressure is needed with a spreader.
