I would not sleeve the barrels due to money. Chamber sleeving should work well for you. A 28 chamber to 20 bore is not a seamless transition. The wad does flex to seal it fairly well. What exactly is your concern? Are your chambers bad or is it just roughness in the forcing cone area? When in doubt ask the expert like Mennington to evaluate your barrels. Sounds like he might be able to clean up the forcing cone area and extend the chamber sleeves to restore area.