The Cosworth CA2013K that was in the MR02 was respectable, but not completely at the F1 cutting edge. The engine was 12kg heavier than the next heaviest in the Ferrari, but the 056/Mercedes FO108x/Renault RS27-20xx were all within half a kilo or so of each other at the 95Kg weight limit for an engine. The Cosworth was slightly more thirsty, it required .130 to .170 of a kilo of fuel per lap, which is about 25kg more over a race distance there. The Cosworth was there or there a bouts in power output, but it was down by 15hp, not much, but so far it is about 0.4 of a second down on the manufacturer units. The last thing is that the Cosworth engine was slightly less drivable, but that is a subjective term. The Cosworth has plus points as well, the engine was easier to work on and install, it was also easier to cool and had a good torque band.
The Cosworth was a good power unit, but if Marussia had a Renault, Mercedes or Ferrari unit yes they would have been able to beat Caterham lots more, but the chassis is what Marussia needs to focus on, there is more time to be found there, however any one saying that the Marussia chassis is a dud for manufacturing reasons are on cloud coo coo, as the Marussia chassis are manufactured in the same place as Force India's at EPM Technology. Marussia just need to focus on more refined Aero development and better quality upgrades not quantity. As long as Marussia chip away at the incremental gains they will be in good shape this year.