The performance gain will be in upgrading your speed unless you need to run a router behind your RG
there will not be a gain unless your computer can take use of 802.11ac and then it will not be any faster than your speed in which you pay for just a longer coverage area and throughput since it is beaming.
A new router isn't going to help you out too much as even with the outdated 802.11g the throughput is stll 54Mbs. So even at 24Mbs your 802.11g will be right there with you. I would say, upgrade both because there are benefits of having your own router, but increased Internet speed is not one of them unless you are a Gigapower subscriber. For me having increased local network speeds is important, but I am certainly not in the majority.