Maximum Switching Speed Chained 74HC595
It would appear that you intend to operate these 13 shift registers using software to bit-bang some GPIO pins of a microcontroller to operate the SER_IN, SER_CLK and REG_CLK of the 74HC595s. It would be unlikely that in this scenario that you would be able to generate the control signals fast enough to exceed the capabilities of the shift registers.  

From a pure analytical standpoint, if you can build the circuit to ensure that the SER_CLK signal is essentially the same signal to all of the shift registers then the maximum shifting frequency will be based upon the same specifications of just the first shift register. If there is delay of the SER_CLK getting from shift register to shift register due to long signal traces then you would want to total up that overall delay from end to end and use that as a guide to understand the fastest positive edge to positive edge to next positive edge of the SER_CLK.

I had a recent project there I operated a series of 8 such shift registers driving a total of 64 LEDs in an 8x8 matrix. I was using a microprocessor running at 22.1184 mHz and writing the tightest code possible to bit-bang the control signals I never even came close to overclocking the shift registers.

You will have to use very careful design with handling the high current paths of your MOSFETs. If you get significant GND noise it could impair the signal integrity at the shift registers and cause them to not operate correctly. Do keep in mind that this type of noise and its ability to disrupt proper operation of the shift registers will have very little to do with how fast you operate the shift register control signals.