Pros/Cons for using the carb diaphragm/air jet cover plate?

I've been rebuilding my XJ650RJ's carb, and found (along w/ several other problems...) that their were no air jet plates. After perusing the forums here today, it appears that these plates weren't necessarily used on all Hitachi's...I'd initially assumed a previous owner had removed them as a 'performance improvement'; so I'm trying to surmise if I should use them. I got a free set of them off a 750RJ's carb set, so they are available to me. Only thing I can think of is that they stabilize the air pressure for the air jets...

And while I'm here...with my rebuild, I needed a couple jet needles...and the place I went to didn't have any 650 needles (Y-10) in good shape; but did have a whole set of 750's (Y-13) which I got. I'm using my original springs, and no shims (the 750's had a shim, and lighter springs); so I'm wondering if you think that would have a big effect? What does the Y-1* indicate; a taper/slope or something? The needles certainly looked the same...

 

Welcome first off, you happened to find some of the best XJ carb people around.
As for the covers, I have them in place on my units but I also understand they were omitted in later models with out a change in jet sizes, leading me to believe they are unneccessary for performance. Take your pick, run them with and without and see what you notice. Report your findings.