If I get time I'll scan up the article on the Tomei built R33 RB25DE with the hex throttle treatment that beats GTR's and revs happily to 10,000 RPM. Although that engine had a lot of work put into strengthened internals too.
One of the reasons people do this is because of the different racing classes. For example in Australia, a R32 RB20DE GTS is eligible in the 2.0L production class racing. If you had a RB20DET GTSt it you get a 1.6x penalty, making your estimated capacity 3.2L and meaning you'll have to compete with V8's, GTR's and other larger capacity cars where you'll have no advantage. If you had a high revving RB20DE though, you'd be king of the 2.0L class (well close to... an S2000 would still kick your arse).