edit* The WR times list was updated after finding faster times at the In The Sky SMK site. WRs that don't appear on Sami's site are in blueI visit the SMK site of donnki (Masakazu Goto) and monchuck occasionally, and they have this page which shows the potential limits of each course for 5-lap and 1-lap (well, I'm assuming they are potential course limits). I'm not sure how they determine what the course limits are, but I enjoy comparing the current world records/fastest times to the potential course limits at donnki and monchuck's site. Keep in mind that the limits at the site are for Time Trials with NBT driving, and that sometimes the strats used in the current world records differ from those that are shown at their site.
Here is the section of their site:
http://f31.aaacafe.ne.jp/~smkt/capture.htm"Translated" to English:
http://babelfish.altavista.com/babelfish/trurl_pagecontent?lp=ja_en&trurl=htt...Here's a list of the potential course limits compared to the current fastest times that I know of: (unfortunately this is only for NTSC

I think it'd be awesome if there was the same thing for PAL, as well as potential course limits for Non-NBT driving)
MC1 =
0'55"80, 10"90 -- current WRs:
0'57"36, 11"05 -- difference: 1"56, 0"15
DP1 =
1'09"50, 13"80 -- current WRs:
1'10"88, 13"80 -- difference: 1"38,
0"00GV1 =
0'58"95, 11"65 -- current WRs:
0'59"92, 11"80 -- difference: 0"97, 0"15
BC1 =
1'25"70, 17"00 -- current WRs:
1'26"76, 17"19 -- difference: 1"06, 0"19
MC2 =
1'01"50, 11"70 -- current WRs:
1'07"21, 13"07 -- difference: 5"71, 1"37
CI1 =
0'55"90, 10"60 -- current WRs:
0'56"57, 10"74 -- difference: 0"67, 0"14
GV2 =
0'58"50, 11"65 -- current WRs:
0'59"59, 11"68 -- difference: 1"09, 0"03
DP2 =
1'23"70, 16"70 -- current WRs:
1'24"90, 16"77 -- difference: 1"20, 0"07
BC2 =
1'37"20, 19"40 -- current WRs:
1'39"30, 19"67 -- difference: 2"10, 0"27
MC3 =
1'25"50, 17"00 -- current WRs:
1'26"98, 17"11 -- difference: 1"48, 0"11
KB1 =
0'46"50, 09"30 -- current WRs:
0'48"78, 09"39 -- difference: 2"28, 0"09
CI2 =
1'04"40, 12"70 -- current WRs:
1'06"73, 12"87 -- difference: 2"33, 0"17
VL1 =
0'47"70, 09"40 -- current WRs:
0'48"83, 09"55 -- difference: 1"13, 0"15
BC3 =
1'27"20, 17"20 -- current WRs:
1'28"27, 17"41 -- difference: 1"07, 0"21
MC4 =
1'31"00, 18"10 -- current WRs:
1'33"74, 18"35 -- difference: 2"74, 0"25
DP3 =
1'14"50, 14"80 -- current WRs:
1'18"20, 15"07 -- difference: 3"70, 0"27
KB2 =
0'54"00, 10"50 -- current WRs:
0'57"15, 10"54 -- difference: 3"15, 0"04
GV3 =
1'12"70, 14"40 -- current WRs:
1'13"42, 14"47 -- difference: 0"72, 0"07
VL2 =
0'45"50, 08"50 -- current WRs:
0'48"57, 09"06 -- difference: 3"07, 0"56
RR =
1'23"80, 16"70 -- current WRs:
1'24"87, 16"88 -- difference: 1"07, 0"18