Blow-through turbocharged propane systems are fairly common and quite simple to do from the propane fuel system point-of-view. The Impco mixer supplies fuel based on air flow not vacuum and the fuel mixture is controlled by the shape of the gas valve. I'm not sure how your friend "shimmed it out to 600 CFM" but it sounds to me like he changed the rate at which the gas valve opens. Regardless of what he did, the exhaust should not make your eyes water. If it does, it could be that the fuel mixture is extremely rich which reduces power and burns exhaust valves.

To allow the mixer to run properly in blow-through mode, you need to install a balance line between the mixer and the converter. This ensures that the mixer supplies the correct fuel mixture regardless of the pressure. Also, the octane rating of propane motor fuel is 104 rather than 110 and a safe CR for mildly cammed, naturally aspirated propane engines is around 10:1 to 11:1.

I have no idea how much boost a 351W can handle but you'll probably be much happier if you keep the boost to mild levels, like around 5 psi.

When you ask about problems other than the usual propane-related issues, there are very few. The main problem is that people don't properly build and maintain the engine and then blame the fuel when something goes wrong. Use propane motor fuel (HD-5) rather than commercial heating propane to minimize residuals in the converter and set the mixer for the correct full throttle fuel mixture.