Apparently all the 4 cyls (for the US market) are twin turbo with the only difference between the high output and the standard output being software. The official Mercedes EPC and WIS shows different turbos for HO vs SO but apparently the Mercedes documentation is wrong, at least for the US market.
See this SS thread where member zimonomiz and Mercedes tech Maslin both explain it:
Standard or High output 4cy diesel?
Also note something that every HO owner should be aware of is that the HO tune will automatically de-rate to the standard tune when the oil exceeds the maximum allowable temperature.