Yeah as posted below that post, straight 5s not V6s.
At 200hp per engine, that's plenty for a small DMU. That means 400hp per powered vehicle, of which the vehicles are only 18m in length. Even considering a 3-car with an unpowered centre trailer, that's 267hp per 18m vehicle, compared with for example a 156 which has 285hp per vehicle for 23m. Given that the D78s and therefore 230s are Aluminium, the power to weight ratio of even the 3-car is perfectly adequate. It won't compare that well to the performance of say, a 185, but it'll be plenty good enough considering the 60mph gearing.
I'm not really convinced about the longevity argument, engines transferred from the automotive industry haven't worked too well in the past and they have to work very hard in the 230s compared to their normal load. Most DMU engines operate sub-2000rpm all the time whereas these are clearly working at 3000rpm+, you can hear that from the testing footage. Component reliability, however, isn't necessarily what Vivarail are going for - more ease of maintenance. If there's redundant engines in use and replacing defective ones is far simpler than on regular DMUs, then it matters a lot less how long the individual engines last. What could potentially be an issue is that with the backlash against diesel engines in small vehicles, this particular engine may be discontinued far sooner than the Cummins / MTU engines used in bigger units. How maintenance will fare in 10 years' time is up for debate.