TIR and Ray depth are used mainly for multi-faceted objects like gems, diamonds, rubies etc, it's what gives them the sparkle, simple TIR on what you have with 98 really won't do anything because you have a fairly constrained geometric shape, with not many sides. As far as Premium renders, Corel stated that it would take longer, they make no fuss about it, reasons, the higher the RPP setting the better the quality of render, for things such as soft shadows ( which is slow even in LightWave 6.5) of course in animations it's hard to use soft shadows etc, it goes by so fast the viewer usually doesn't even notice if the shadows are soft or not.But for still images it would be noticable, so I reckon it's just an artist preference thing, like using this brush or that brush.

Do atleast one thing a day that scares the hell outta ya!!