I tried the settings you recommended to increase the drop rate from the gravel sieve but it still doesn't even come close. After 2000 gravel sieved I still only ended up with a total of 6 gold lumps and 1 iron lump. Mining manually produces almost 25x that rate with all ores.
The current setting I have is 0.3 to get 3x the drop rate you suggested but it still doesn't work. I only get about 1/50th of the ore lumps compared to actual mining. This is kind of making it useless to have an ore gen or even use the gravel sieve at all.