Don't worry about the 300 GM nuggets getting passed thru the sluice, they never actually get passed the hand of the dredge operator to even have a chance of being sent to the tailing pile!!!!!
The grizzle bars did not work, they trapped the rocks under and would not release them. I did have success with a sump set up that fed to a small 2 inch dredge but let that go due to the fact it was complex and negated the benefit of a light weight sub surface dredge.
I don't believe I lost any gold that had size to it. All the nuggets got caught in the first or second riffle, the fines are another story. Basically the sub surface dredge is a poor choice for an area that has lots of fine gold. Salt gold or thin flat flakes etc. but can do really well on the chunky nugget stuff, bird shot on up. Just keep in mind not to hog the material and overload the riffle tray, that's where I get goofed up.
I did add a river sluice behind my box at one point and the extra box being wider did help but back to more complex.
My advice is to add miners moss over the ribbed matting and nothing more needs to be done. The rest is a diminishing return situation.
Keep us posted on what you end up with. Also would love to see a pic of your subbie🙂