Bought the white upgrade kit recently, to use with my regular sized Gill Mask (bought in Apr 2020). For me, the notable improvement was the silicone strap. Although the silicone strap is more rigid and less flexible than the previous spandex/nylon strap, it gives a tighter mask fit on your face. A tighter fit leads to good sealing, and hence better protection. The breathability is still acceptable for me, when I cut out surgical face masks to use with the old Gill mask and silicone strap. Though I have yet to use the P95 filter with the PRO mask, I believe the improvement will be more noticeable when done so. If you are still pondering on whether to buy the upgrade kit or not, I would suggest to buy only the silicone strap first. Getting a good mask fit should be your highest priority.
The intrinsic downside is that the mask muffles your voice greatly when in use. Its large size and heavy mass is also part of the product's downside. This is unavoidable, but your safety is more important, right?