The extended won't be a problem on either case. One thing that will happen is the leather case will stretch slightly to handle the ext batt so if you use the slim again there will be a very minor airspace toward the bottom of that case. The "nextel" HD case is made to handle the the ext batt... most Nextel users have that battery.
The Proclip has enough rear space to handle the ext batt as well.
You're cleared for take off!