While I have heard of vacuum plate issues it seems to be, as you pointed out, most common on singles. I have not heard of anyone with multis having the issue. Also as pointed out there is a replacement. I run the FCR carbs mainly because I scored them cheap but they do offer an advantage in flow, here the USA the SCCA does not allow non-butterfly actuated carbs in production classes.......I'm told due to the increase flow and the fixed choke/Venturi size. Under the possible heading of urban myth slide carbs do seem to offer better response than butterfly carbs. Note CV carbs fall under butterfly carbs.
As for additional jetting options that is one of the other features I like about the FCR carbs; I changed the needles to ones with a different taper and L1 measurement to get mine exactly dialed in. Once the car was dialed in it has been just a matter of minor jetting changes. Now there is one downside to FCR.......the little clips on the tops of the needle for adjusting the height, they are quite easy to loose. I use needle nose vice grips and keep extra clips in my jet box.

Tom