I have 2 CM15As and did the F-Connector (actually a BNC for 50-Ohm cable) mod on the first one and clipped the plastic off the top of the 2nd one's antenna and slipped a metal telescopic antenna (with electrical contact to the original antenna wire) over the plastic antenna.
Both of these mods have worked very well for me; however, since both CM15As and their PC's are on the basement level of my house, my vote goes to the external connector mod.
The attached telescopic antenna was easy and improved the reception considerably, but because of where I have it located, it does not cover all areas of my home.
The antenna for the external connector mod'ed CM15A is located in the attic, and covers 100% of my property.
Having stated my choice and reasons, I would recommend someone trying the easy external antenna attachment first before they open the CM15A and start modifying it from the inside.