Hi Cameron,
Using the benchmarks we have for TM4C123GH6PM board I would estimate the context switch time to be around 71 cycles on a TM4C129 board.
(Referring to same computation logic mentioned in http://e2e.ti.com/support/embedded/tirtos/f/355/p/358488/1258438.aspx#1258438)
(Link to M4F benchmarks http://downloads.ti.com/dsps/dsps_public_sw/sdo_sb/targetcontent/sysbios/6_40_02_27/exports/bios_6_40_02_27/docs/cdoc/ti/sysbios/benchmarks/doc-files/M4F_times.html)
Best,
Ashish