1) Is there really any noticeable difference between TIFF and SHQ
mode?

It depends on the camera. In some cameras, finest JPEG is virtually free of
compression artifacts. In some others, it isn't.

I don't know about your particular camera. I guess if you look for reviews they
may give you an idea of the quality of its JPEG processing.

2) Any reason to be shooting under one mode vs the other?

Again. It depends on the camera. But more importantly too, it depends on the
person and circumstances. Personally I only shoot finest JPEG. But that's
because I specifically researched and chose a camera which finest JPEG is
virtually free of compression artifacts, and in-camera processing is award
winning (DIMA 2004). It also enables me to take far more shots, as TIFF file
size can be more than twice or up to three times that of JPEG.