With the Impreza, what you do have going for you is its relatively small absolute size, allowing a single-head good coverage/protection - however, at the same time, you have a VERY "flat" rear, with widely spaced hard-points (the very highly reflective tail-light assemblies. This is a hard call to make, and will likely require that you do some quantitative testing of your own to see what kind of true protection you're getting.
With the front, offsetting the jammer head to one side or the other will somewhat reduce its overall performance, particularly as engagement distances get closer. serg
did some testing on this and the off-side did show PTs at closer ranges. As such, I would base your proposed front-mount configuration based on whether or not you tend to see engagements at closer distances (i.e. < 500 ft.).
Granted, serg's car is a much-larger vehicle, and metallic silver to boot - but still, it's something that's both worth noting and considering.
Unfortunately, yes, recessing the head will cause some performance degradation - this is going to be related to the "field of view" that the jammer head has, especially with varied roadway conditions (i.e. dips/rises in the road, curves, - and how "severe" each case may be, especially combined with off-angle enforcement shots - etc.).
I do understand that in your area, jammers are prohibited - so that stealth is a very important factor. However it may be worth it for you - if you are going to see shorter-range engagements - to either locate the jammer centrally, under the front number plate (in the front radiator opening, making sure that it is "flush" to insure best performance) or obtain one of the many aftermarket front grills available for your vehicle, and see if one would allow you a more "bumper-flush" and centralized mounting of your front head. These options may have some aesthetic/stealth compromises, so you'll definitely have to work out what's good for you.