First, a word of caution: sorghum, while it is a grain crop, is also a "cane" crop, like sugar. Oftentimes, if you see "sorghum syrup", it is made from the cane juice of the plant, NOT the grain.
Second: you can sub sorghum syrup for brown rice syrup 1:1. Honey and agave you would probably want to use a little more, not because they'll lower the OG but because they will attenuate more fully. For example, if you had an OG of 1.053 from an all-honey batch, it might go all the way down to 1.004 with a yeast like Fermentis US-05, whereas an all-sorghum batch might stop at 1.012. Belgian candi syrup is rated at 1.032, meaning it contributes 32 gravity points per pound of syrup added to a gallon of water (ppg, or points-per-pound-per-gallon). That's a little bit less than your average Briess sorghum syrup, which is usually (IIRC) rated at 1.037.
Rice syrup doesn't really avoid the twangy taste, it reduces it a bit but it's definitely not a huge improvement. Candi syrup helps a little bit but for lighter beers you can't really use enough to do much good. Honey can help, if you use the raw unfiltered stuff, but then it just tastes like honey. Agave is nice, I'm still trying to figure out exactly what it's bringing to the table, but for what it's worth, the only extract beers I brew these days contain a combo of sorghum extract, rice syrup, agave nectar, and a light honey like clover or orange blossom. It's not exactly "beer-like", but it's nicely balanced and doesn't have any prominent off-flavors.
Finally, stay AWAY from molasses--that stuff really amplifies the metallic sorghum twang. Using plenty of yeast nutrient to increase FAN seems to help, as does adding the sorghum at the end of the boil.
For more info, see this blog post I did: http://beyondbarley.blogspot.com/201...act-beers.html