Now Brian, it kinda makes sense as per Andy’s statement about the RF level control and trying to reduce the drive to produce such a low output value from PA and potential instability?
(:>J
PS: Now on my second cuppa tea after my bowl of porridge ‘Goldilocks’ 
I suspect the question re the way the FT 857D behaves is related to the known issue with the IC 910. When you put the output to say 10W and hit the PTT the 910 may briefly output 100W and then ramp its power back to the set 10W. This can damage a transverter if it cant handle a brief burst of 100W.
An FT817 is unlikely to produce 100W but a FT 857D can.
Compton
Thanks, Jack. Compton almost put his finger on it. And Andy pointed out the issue I’m talking about. 
As most modern TRXs the FT-857 regulates P/out with the ALC. This may result in spikes of full power at the beginning of a transmission, what would be 50 W on the 2 m band. 10 dB more than the specs of the sg-transverter, though the FT-857 can be set to 5 W and even lower if you adjust the lower P/out in the service menu. I don’t want to be the first to roast the sg by using a high-power rig.
On the other hand I don’t own an FT-817 but an old FT-290R (5 W w/o PA, 25 W with PA, -3 dB against the 857). I would rather have only one rig to schlepp up the summit for both, 2 m and 23 cm.
Yes, I know, driving the sg with a FT-857 is like shooting sparrows with canonballs.
Ahoi
Pom
It is possible to make a limiter that removes the spikes. Thinking on my feet may be back to back (series) diodes. That conduct at say 6W or so. In normal use they’d be high impedance. I’m sure this issue must have been solved by transverter users using other ALC limited QRO rigs.
You’d gain bonus points from me for using an old school 290 with a very modern transverter. 
The FT8*7 series has three power levels that can be selected in each set of bands. The 6m, 2m and 70cm bands each have their own settings.
For each power level, there are two settings, one being the output power and the other more important one being the RF driver gain.
If the 2 watt level has been configured correctly, the radio won’t be able to output a 50w spike because the drive level will have been set appropriately, so that the radio will only just make the power level set as the maximum ie 2w.
If the user performs a super master reset, all the control levels are set to max. In that situation, all power levels are the same. If the power is cut back using the ALC setting, leaving the gain set to maximum, the 857 is then capable of a 50w spike followed by the gain being reduced by ALC to the preset output limit. IMO these power controls are more suitable for FM modes than SSB. Using the radio to drive a transverter is outside its designed application, so some extra research is advisable.
The bottom line is that if the radio has been configured using the factory procedure, and you set it to a lower power level, eg 0.5w or 1w in the case of the 817, you should not get any power spikes higher than the nominated level.