i know a lot of SC on SG.
I used two SCs in this video (16"50 first lap, first try

):
http://www.termopilucco.com/mksc/nontick/vids/mksc_nt_sg_fl.aviThird shroom is wrong, maybe you have to do it on the left of the final path or maybe at the start of the lap (try to cut some road).
I'm not sure both the SCs i used are allowed.
Edit: a better use of the third shroom (15"71, second lap):
http://www.termopilucco.com/mksc/nontick/vids/mksc_nt_sg_fl2.avi Do not laugh for the first section, i've made the video the first time i got the 3 SC's in a row

Edit2:
the real big SC of SG that make every player crazy (this is my old and only one 'successful' attempt after 2 hour of tries):
http://www.termopilucco.com/mksc/nontick/vids/SkyCircuit_badSc.aviThe last big SC it's not forbidden by the track rules, but a general rule say that major SC's are not allowed.
So i dont know really...
edit: the old GBA SP video output is way better than the DS one for my old videocamera...
edit2: first SG video replaced with a faster one (14"84) and second SG video deleted
edit3: first SG video replaced with a faster one (13"63)