I use Bowser, and yes I jump over all the yellow jumps !
I play only on 150cc, so I can't give you times for 50cc... But I can do one Cup on 50cc, and I'll send you my times to have an indication.

About coins, I did once *** with only 96 coins, but it's very, very difficult !

I did 4 perfect races (I wrote the splis in the topic "Three stars" I think) and it's simply too hard !

With 120 at least it's "easier".
At KB1 I already did some races over the water (as on Time Trial), but the most often I take less than 20 coins and it kills me for ***. Once I reached only 3 coins at KB1 (but I did a 0'41"), 3 others good races, but finished with *...

So, I always drive on the sand at each lap.
Oh and at MC3 I always slow down at the hairpin... Possible to pass it at full speed, and reach the boost on the sand after, but it's hard !

I'm pretty sur I'll be able to upload my vid tomorrow.