From attempting to calculate the best way for various movements through the St. Louis area, the only Missouri to Illinois through interstate movement that sees any mileage savings from using the beltway appears to be I-70 to I-70. From the I-64 interchange at Wentzville to the I-55 interchange at Troy:
* 53 miles using I-270
* 53.5 miles if MO 370 is used to access I-270 (+/- 1 mile; can't recall the last quarter mile marker on MO 370)
* 56.5 miles if traffic stays on I-70 and uses I-170 to reach I-270.
* 61.25 staying on I-70
* 60.5 if I-64 is used on the Missouri side
* 58.25 if I-64 and MO 364 (when completed) are used to reach I-270 (assuming MO 364 is 20 miles long when finished)
* 63.25 if I-64, MO 364 (when completed), I-270, and I-64 are used on the Missouri side
The distances are also probably off since IDOT doesn't have a nice 1/4 mile marker map like MoDOT has:http://www.modot.org/stlouis/links/docs/StL_Mile_Markers_204.pdf
When the Great Lemon Bridge for I-70 downtown is finished, the staying on I-70 route will see the distance reduced by 1.6 miles to 59.65.
The I-55 (MO) to I-64 (IL) movement is almost equal distance wise via the PSB or I-255, being only about 3/4 mile shorter via the PSB.
I'm not going to estimate travel times yet because
1) Difficulties in using the PSB, especially the I-55 ramps
2) Most drivers ignore the speed limits anyways
3) With regards to #2, Illinois generally seems to have heavier enforcement of speed limits.