No. I believe it’s the Sheetcam Mach3 Post that’s the issue. Their post-processor assumes seconds because that’s the standard Mach3 install.
So you either have to alter the post-processor so you have a ms version or trick it by entering ms in Sheetcam (but telling it those are seconds) and then telling Mach3 in the General Config G04 setting to be ms. Sheetcam will not do the math in the post processor to convert your entry to ms as it’s doing by default but it is ms and Mach3 will be happy. Clear as mud eh? 
Here’s what’s happening in Sheetcam using their Mach3 post processor.
Sheetcam set for seconds
Enter 1 second
GCode is “G04 P1”
Sheetcam set for ms
Enter 1000
GCode is “G04 P1” which is not what we want. But it’s doing that because the post processor is coded to output in seconds.
The hack:
Sheetcam set for secs
Enter 1000 (milliseconds)
GCode is “G04 P1000” which we do want because our implementations of Mach3 is going to treat these as milliseconds.
So that’s the quick & dirty of things. You would think that Sheetcam’s post would output stuff correctly since we tell Sheetcam if we’re using seconds or milliseconds. But the post doesn’t do that.
I think I have a millisecond based custom Sheetcam post processor but before I upload it so people can use it I’d prefer another set of eyes on it. If you’re a Sheetcam user and want to test my post processor, let me know.
What I’m doing in there is changing the G-Code generation step to take the numeric portion of the Sheetcam G04 output and multiplying by 1000 to offset what Sheetcam has done when you specify ms in your Sheetcam settings (it’s converting to seconds and sending that to the post processor - mine simply reverses that).