The memory controller is aboard the CPU - the memory speed is set by the CPU itself. What model CPU do you have?
Not without replacing the mainboard - to which it is soldered. And that will cost you likely over $400 just for the part - and will void your warranty.
Userbenchmark shows certain good scores with XPS 15 9550 i7 and G.SKILL 2667 2x16GB RAM.
Most claim though that 2400 is the max that 9550/9560 will do. No idea if a hack is involved above. The processor and the RAM negotiate the clock and the latencies automatically, you can only see the outcome (check BIOS, CPU-Z, Userbenchmark). Mind RAM may run faster also at 2400 if it works with a lower latency. And that while faster RAM will produce the expected advantage in specific benchmarks and with particular RAM-intensive programs, but the overall performance is likely to improve by less than a percent, unlike going from a HDD to an SSD which is sensed by anyone, so there are likely better ways of spending money than on "premium" RAM.