I've not heard of Mitsuba before. Facet pumps are popular and compact. Could I ask why you prefer Mitsuba. Are they better than facet? Do you use some kind of roll-over valve or automatic cut-out to stop it pumping in the event of accident?Advice above is good. However, I've been fitting an electric pump in the engine bay to all four of my 105's for the past 12 years and thousands of miles in each, with no issues whatsoever, (see above a Mitsuba pump just fitted to a Spider, my latest acquisition.)
I personally wouldn't keep the mechanical one, but you obviously can if you prefer.
With an electric-only set-up, you can use a hidden on/off switch to use as an anti-theft device if you like.