Even an Isp of 240 is not all that high.  This page:<p><a href="http:&#x2F;&#x2F;www.astronautix.com&#x2F;props&#x2F;n2o4mmh.htm" rel="nofollow">http:&#x2F;&#x2F;www.astronautix.com&#x2F;props&#x2F;n2o4mmh.htm</a><p>Lists a bunch of engines that have Isp values in the 300 range.<p>SpaceX tends to optimize for total operational cost, rather than performance, so I wouldn&#x27;t be surprised by the tradeoffs they&#x27;ve made to create a reliable and reusable system.