All of the PA30 I have flown have had marginal performance on one engine as is common in most light twins. I did an LPC on a guy once in one and on the EFATO it really was a case of the second engine just there to take us to the scene of the crash.
FWIW, the POH OEI sea level climb rate of the 3725 lb PA30 (with tip tanks) is just 170 fpm at MTOW. However, if it is correct that the intention was to pick up two pilots and depart, then it's likely that the operating weight was (at least) 300 lb less, and one would expect a OEI sea level climb rate of 300 to 350 fpm.