what we need to know are as follows:
application of the vacuum plate
external forces or weights resisted by the vacuum plate
assuming the vacuum plate has an infinite number of perforation,the max force that it can hold is
(8*.14504*3=) 3.48 lbs. which is not much
I suspect that the pump is operating close to the max static point based on the information of the small vacuum plate that you are using.
a larger pump will probably not offer a significant increase in holding power of the plastic sheet.