Grill cloth is cloth and you're placing it right in front of your speakers...there is an impact, no way around that.
Ever heard the line about pulling a blanket off the speakers?? The old style grill cloth is like putting the blanket back on!
Marshall used to use a cloth in the mid 60's (65 to 68 give or take) that was very thick and dense...as grill cloth goes it's not what most people really want...it really messed with the sound by rolling off a LOT of the high end of the amp and speakers.
Most of the time amp builders and speaker cab builders consider this a bad thing but if you want to really nail the sound of a mid 60's Marshall you'll find that if you use modern (post 1968) grill cloth the sound is far brighter and more in your face in the high freqs than the original amps are/were, when you kill off some of the highs then all the sudden the mids are very focused and up front.
Call it cork sniffing if you want but it does make a difference.
As for the cost of the Collins "True Bluesbreaker" cloth it's expensive to make and since he's having it made in very small batches there is no big order discount so the bottom line is it ain't cheap.
I'm not saying you have to have it I am just saying that it does make a difference.
A JTM45 with it can sound good and a JTM45 without it can sound good but both amps will sound different.
Have you ever even bothered to read their website or are you just making up your mind on a product based on it's cost??
http://www.bluesbreaker-pinstripe.com/