The beta cluster is currently unsuitable for testing code for many performance problems, because it is all on VMs. It generally won't tell you about cachebusting either, for several of our caching layers.
Several people are working on Vagrant roles that will help developers test in more realistic environments in dev environments, with million-row databases and warm caches. But CPU and other constraints won't be realistic.
So - as one can see in https://www.mediawiki.org/wiki/Performance_profiling_for_Wikimedia_code - right now, many interactions we can only predict roughly until the code hits production.
It is not possible to have a testing cluster that exactly mirrors production. And of course it is the developer's responsibility to know the constraints of the production system and know how her code exercises those systems. And with heterogeneous deployment, it's *possible* to notice some problems while they're only affecting less-trafficed wikis. But some people have expressed interest in creating a more realistic environment to test/predict how efficient code will be when deployed to very-high-traffic wikis, especially with unique configurations.