GSMArena has run some benchmarks comparing Windows Phone 8 GDR3 to Windows Phone 8.1 Developer Preview.
They found significant improvements, primarily tied to the improved browser rendering engine in IE11.
The Basemark OS II benchmark shows improvement in all devices, while Browsermark 2 shows up to 30% improvements in IE11 vs IE10 Mobile.
GSMArena also noted improvements in the SunSpider test of around 5% and in the HTML5 browser test.
The benchmarking shows that beyond the new features there are even more reasons to upgrade to the new OS when the final version finally becomes available in June.