You're better off extracting from the syrup than from those. Not to mention that the DXM content is far below that of Robo.
You never know what binders they use that may end up in the final product if you do decide to extract though, so you may not get pure DXM like you'd want.
Extracting the syrup is pretty simple and not very time consuming and you get a nice pile of (pretty much) pure crystals.
As for the nausea, have you thought of the possibility that these might swell a little in your stomach? Times that by 50-60 and the nausea is going to be pretty bad.