Its a bit of a confusing post Ben. The EX3 does not shoot XDCAM HD422 1080p 24fps ... at best it shoots full raster XDCAM HD at 4:2:0. Regardless of the frame rate and frame size you choose, the highest quality setting on the EX3 is the "HQ" setting, which records using the 35Mb/s VBR (variable bit rate) codec .... so you are looking at about 4 minutes per gigabyte, although depending on the subject matter the variable bit rate compression will offer some advantage.
Re editing in XDCAM EX/HD, I'e had great success editing natively with that codec on a MacBook Pro, however I would advise setting your interim render codec to ProRes (that's a good combination of taking advantage of the smaller source clip file size / data rate and FCP's camera native codec support for XDCAM EX in the timeline, together with the benefit of the quality and speed of ProRes as a render codec) .... and
never master back to the XDCAM EX/HD codec (mastering to ProRes is my choice). Depending on the power of your laptop you might consider the option of editing directly with the XDCAM EX source clips in a ProRes sequence. This is an approach favored by many with more powerful systems and does confer some advantages if and when mastering out to ProRes eg all interim renders can be used for export.
Hope it helps
Andy