That really depends on what version you're using now, and what exactly you mean by scalability.

If you're using 9.1 or older, you may see a significant improvement in OLTP workloads on many-core machines (making it linearly scalable to >64 CPUs). This happened in 9.2 (i.e. ~2 years ago).

The main improvement in 9.4 I'm aware of is the GIN fastscan, which significantly improves performance of applications using GIN indexes (e.g. full-text).

Of course, there are many other performance improvements on various places - the principle is not to make the new version slower.

Some interesting numbers were presented in this talk at pgconf.eu 2014, including 9.4 beta (but there should be no significant differences): http://www.slideshare.net/fuzzycz/performance-archaeology-40...