No, you'll be fine.
Opening up the rear edge of the bonnet isn’t really going to let any rain or “car washing” water in.
Any rain or “car washing” water that runs down the windscreen e.t.c. will still drain into the channel/gutter (under the windscreen wipers plastic scuttle panel) that was designed to take this run off water.
I assume you’re taking the normal precautions of covering the spark plug valley in the rocker cover with some sort of spark plug cover.
Like I say in the original thread about these spacers, I have had these spacers fitted to my (daily driver) car for way over a year, I have removed the plastic panels on the underside of the small bonnet vents and run with no spark plug cover, no problem at all.
Steve