You ahve some nice skis picked out there. What are you on now? How do you like them?
I own the 5* T50 (2 year old model) so I am sort of biased. The skis rip. 168cm is the length for short turns and speed BTW. However thay aren't forgiving and have a ton of energy and rebound. And the stiff flex can be a bit much in some of the icier moguls we get here now and then. I love these skis for everythign else, including, crud, less than 8" soft snow, and icy groomers. I skied them on the rutted up nastar track on exhibition today after their practice. These skis are awesome on ice. I was flying down there just carving it up in whatever shape turn I wanted and was having a great time. The 5*s take everythign you put into them and give it right back to you. If you take an edge and stay centered the skis are totally stable in variable conditions.
I think the RX8 is less stiff (in the tip to tail flex) than the 5* and maybe easier to handle. Also, check out the Elan S10 as it is a slightly softer version of the S12 (1 metal sheet instead of 2 i think). That may be better for you.
If it were me, I would demo all 3 (or 4) and then make my choice. You need to get the right length as well. All things being equal buy the 5* and take an extra private lesson with your savings.