I just got an SDRPlay and an MFJ-1708SDR transmit switch. I use the the PTT on my Flex to key the amp, so I am wondering if the RF sense is fast enough to protect my SDR radio?
In your description, you state that your Flex controls the amp. Be aware that the MFJ switch is only rated to 200 watts PEP. Exceeding this limit is an invitation for failure.
If wired properly and used within its specifications, the MFJ switch should protect your SDR input. I have no experience with this switch but I would recommend that you pay attention to sequencing of all devices in the RF path at the start and end of a transmission. At a minimum, the Flex should be programmed to delay its RF output on PTT assertion for a sufficient time to allow the MFJ switch to transition to transmit mode prior to the Flex emitting RF. The amp also needs time to switch so the Flex delay should be at least as long as the longest switch time of the two and then add a 20% safety margin.
Ideally you would key the MFJ, wait a sufficient time for it to switch, then key the amp and wait for its R to T switch delay, and then finally allow RF from the transmitter. Reverse the sequence upon release of the PTT. This scheme ensures that any transients or oscillations from the amp cannot find their way into the SDR input. There are add on sequencer circuits designed to perform these functions if your existing equipment does not directly offer sufficient sequencing features.
You could also add an input protection device, such as those sold by DX Engineering and others, to the SDR input as another layer of protection.