I tried searching but I only found info on the midrange diameter being 6.5". Can anyone tell me what's the mounting depth of the OEM speakers? Or if there's possibility to go deeper without fouling any mechanism?

The ones I'm looking at are 75mm (2.95") deep.
http://www.hertzaudiovideo.com/Doc/H...y_HSK165XL.pdf

 

There will be a problem with the top part of the original mounting cover.
This can't be closed any more.

I used a diecast ring with 10 mm depth and used foam to prevent that the speaker play into the door.
I would hardly recommend to dampen the doors very good, because the Hertz speakers can play very loud and very deep. If you have the possiblilty i would hardly recommend to cut them of with an active x-over.

There will be no problem with the depth of the speaker, or the width of the magnet.

But to be honest. If you want to get the best results it's best to build a new very massive mounting for the speakers.