Before you pull the trigger on the 105 CPL, you should consider the wonderpana system:http://www.wonderpana.com/
I haven't used it, but it's basically a Lee-style system with much
larger filters so that the vignetting problem is a non-issue for UWA focal lengths. If you want to stack stuff and have a CPL at 16mm, this is the system you should look into.
That said, I love my Lee setup. It's well built and has industry standard sizing so I am not married to first party 4x6 filters or CPLs. And it's flexible and powerful. I can double my ND grads on harsh sunlight, stack an ND grad with a 10 stop ND, and now with a front ring the CPL is an independent consideration if I need to manage the sky (only for longer FL) or reflections (at any FL). The only time I need to juggle/think is between 16-20mm, and that's fine by me.
Thanks for the advise, but I have been using the Lee system (so far, the Big Stopper and a set of hard ND grads that I have also been using as straight NDs - planning to add real NDs and some soft grads soon). I have been enjoying the system. But, I have been limping along, putting my 77mm CPL between the lens and the Lee. It's nearly impossible to rotate the CPL then set the grads (at least without getting finger prints all over). Hence taking the next step and adding the 105 CPL ring and CPL. Like you, I will need to deal with the 16/17 to around 20mm.