'loose lower shifting', what's that? I suppose the shifter can get loose if someone was abusing the stick. If there's a experience driver behind the wheels, the stick ususally doesn't get abused and will always feel as tight as a Vergin.
The only thing I know of that'll change the shift action and also tighten up a loose and worn-out shifter is a short-shift kit. Personally, I think shorter shifts would be nice, especially considering that longer shifts in the Yaris isn't that smooth (but till good). So, if it's going to be slightly notchy anyways, might as well go with the shorter shifts. But, with the shorter shifts, you need shorter clutch travel to balance it out, e.g. the Celica GTS has a perfect shift length that matches a short clutch travel....so, execute fast shift in that car is a breeze. In fact, the Celica's sporty setup makes for a smooth shifting when you're going extreme and gets hefty/notchy when you're cruising. So, there's trade-offs, but not as much as some other cars (e.g. RSX-S). Anyways, if you're going to get a short shift kit for the Yaris, but keep the the clutch travel the same, it's probably going to make for some awkward shifts when doing things quickly. They've got to match-up...unless, your left foot is faster then your right hand, but I've found that the shift-hand is usually the faster.