I think tinting is as much of a liability as an asset in this consideration, as it might draw attention to the vehicle with thieves wondering what it is that you have which you don't want seen? Dark tinting is common and doesn't attract attention in desert climates, but will attract attention in places where it's not necessary and isn't in common use such as you'll find in London.
There are several approaches to security. One is to not have anything to steal (removing the cam every time you park). Another is having a stealthy install (making the cam hard to notice from the outside). Yet another is having a car alarm (which you hope someone pays attention to if it gets activated). Still another is using a cam with remote sensors and the main unit well-hidden (this depends on the thief understanding that what they see isn't a whole cam and thus has no value to them). We'll presume having no cam isn't an option although they can't steal what you don't have.
An adjunct to all this (and perhaps equally important) is parking in the safest place. That can be achieved through location, or by always parking where a clearly more lucrative target will draw the thieves to it instead of you, or perhaps both of these. A have a small sign at the back doors of my van next to a visible cam stating that whoever is there is being recorded and it's being sent to the 'cloud' which isn't true, but serves as a deterrent if they think it's true. More than a few times I've noticed people seeing that cam and reading that sign.
Window tinting can reduce a cam's video capabilities at night considerably, and at best requires some experimentation to find the best exposure settings. I wouldn't bother with it if this was my only consideration; rather I'd recommend (worst case) an easily removable cam or (normally) a choice of cam which allows a stealthy install that makes it tough to determine it's presence.
Phil
