Ultimately, a macro nexus provides enough energy to keep a building under chronoboost ~50% of the time. Each chronoboost cuts 10s off of production time, so any production structure will receive a 25% boost to productivity. On two bases with a macro nexus, you can constantly chrono one forge and 50% chronoboost the other. That will get you a +3 attack or armor at 380 seconds as opposed to 430 with 50% chronoboost.

The verdict, a nexus is nowhere near as productive as adding another production structure and significantly more expensive. It can cut 50 seconds off a +3 timing attack, but you have to lay the nexus at a point where you're still building probes and those 400 minerals will eat into your economy meaning that with the cost of the super-fast upgrades, you'll likely not have a lot of units to take advantage of it. Not to mention in PvX, 2-base double forge gets your upgrades so fast anyway, that the 50 second advantage likely won't counteract your unit disadvantage.

The only concrete advantage a macro nexus has over more production structures is if you're using multiple kinds of production structures. The extra flexibility of chronoboost gives you flexibility in shifting your production priorities without leaving idle T2 production structures. Even then, the small boost to production a nexus gives is still vastly outweighed by its cost.

TL;DR 50 second cut in +3 timing. 400 minerals on a nexus means 5 void rays from a stargate instead of 4. 150/150 for an extra stargate means 8 void rays in that same amount of time. Not worth it.