[J_E_F_F]

[*]The performance of the SDHost driver suffers significantly at higher core_freq with force_turbo disabled. Probably best not to use sdhost at higher core_freq unless force_turbo is enabled.

I just did a bunch of testing on my own Pi2. It seems this is no longer the case
I achieved my fastest result with a Core of 500 and SDHOST at 100

Code:

arm_freq=1000
core_freq=500
avoid_pwm_pll=1
dtoverlay=sdhost,overclock_50=100
over_voltage=1

I have 32GB Samsung Evo Class 10 Micro SD cards. Tried with and without setting sdhost, and also only setting sdhost.
........................... Read ............ Write
Stock (all) ............ 16MB/s ....... 12MB/s
800 / 500 ............. 16MB/s ....... 12MB/s
sdhost100............. 28MB/s ....... 16MB/s
1000 / 500 / 84 .... 28MB/s ........ 18MB/s
1000 / 500 / 100 .. 32MB/s ........ 18MB/s

To test, I used a modified version of the Overclock stability test at the bottom of the page here http://elinux.org/RPiconfig (had to remove SUDO, and only ran the read/write test 1 time instead of 10, didn't want to wait 5 hours for the test to complete. Took from 20-30 minutes each time only running it once.

If anyone else wants to try it, I just changed seq 1 10 from 10 to 1 seq 1 1 and removed sudo from the read line. I use openelec and it would not work with the sudo command.