For one the LPW4 engine on the 803 is ~1,800cc. The engine on the 003 is ~2,300cc. That itself isn't a predictor of power output but I can say the higher rotating mass of the larger engine will certainly help it handle surges better.
I haven't done formal measurements but from looking at the two, virtually everything on the MEP003 is just beefier.
With my home here I was able to run the MEP003 up to around 14kw before it seemed to suffer. I stopped here because I was becoming concerned about an electrical overload. The engine while showing it was loaded, was still keeping up.
My MEP803 which runs very well gets unhappy around 13kw. RPM starts dropping as you go above that level.
Not a scientific test and I didn't do that stuff back to back but overall I find the MEP803s I go thorugh from auction will do well up to around 12.5kw where the MEP003 I have here will do over 14kw.
I offer this strictly as my observation.