Some triplestores (Virtuoso, Jena SDB) are based on relational databases and simply provide an RDF / SPARQL interface. So to rephrase the question slighty, are triplestores built from the ground up as a triplestore more performant than those that aren't - @steve-harris definitely knows the answer to that ;) but I wager a yes.
Secondly, what features do triplestores have that RDBMS don't. The simple answer is support for SPARQL, RDF, OWL etc. (i.e the Semantic Web Technology stack) and to make it a fair fight, its better to define the value of SPARQL based on SPARQL 1.1 (it has considerably more features than 1.0). This provides support for federation (so so cool), property path expressions and entailment regimes along with an standards set of update protocols, graph management protocols (that SPARQL 1.0 didn't have and sorely lacked). Also @steve-harris points out that transactions are not part of the standard (can of worms) although many vendors provide non-standardised mechanisms for transactions (Virtuoso supports JDBC and Hibernate compliant connection pooling and management along with all the transactional features of Hibernate)
The big drawback in my mind is that not many triplestores support all of SPARQL 1.1 (since it is still not in recommendation) and this is where the real benefits lie.
Having said that, I am and always have been an advocate of substituting RDBMS with triplestores and platforms I deliver run entirely off triplestores (Volkswagen in my last role was an example of this), deprecating the need for RDBMS. An additional advantage is that Object to RDF mapping is more flexible and provides more options and flexibility than traditional ORM (also known as putting a square peg in a round hole).
