Guff,
Walkers modified mufflers usually only have the factory screen intact I don't believe that they put any in the pipes atleast mine does'nt have it and I have not seen any like that..You will notice a difference in power,I think you will be alright without the screens in, my saws have the factory screen and I use them daily without incident and I'm sure so do many others.I did notice a difference in power when I added the muffler to my 385 but if you really want a difference in power I would suggest emailing one of the many great saw builders here on the site such as Dan Henry,Ken Dunn,Dennis Greffard,and all the others they have very reasonable prices and they all do excellent work and you get big gains,since I added the muffler I sent the saw to Dan Henry and had it Power Ported and I can tell you it's alot faster than my stock 3120XPG the saw just flat out kicks a$$..
Later Rob..