While this (and a few other things) may be tougher in V5 than they were in V4, there are so many other ways that V5 shines over V4. I can show you where we have saved 6 weeks over V4 on specific design tasks.
It's easy to say "I can't do this in V5 like I can in V4". The real challenge, and the real way to get the job done, is to ask "What is the end job that I need done", and find out how to do it. You will usually find that you can do it a different way in V5 than you did in V4, and you can save much time doing it that way.
In your case, Do you really need the support geometry mirrored? Or do you just need the final solid? Can you find a way to make the appropriate changes on the "original" part so that the final, mirrored part looks correct? Can you simply start with the original part mirrored and make some pads, pockets, holes, etc. to come up with the final part? Is there some common, fundamental geometry that could be put into a 3rd CATPart document, and then use that to create your left and right hand parts?
Much of the key to the power of V5 is not in simply creating the geometry. Building solids is pretty much the same in all CAD modellers. Some have these tools, some have those tools, but it all boils down to the same. The real key to the power of V5 is in it's ability to modify and re-use the design intent of your original designs, so that follow-on designs can be created faster.
I'll get off of my soapbox now.