Powering multiple DC gadget(Routers, switches, RPis etc) using a single smps will be efficient than multiple power supply?
No, it will not be worth it. All power supplies have a certain power at which they're most efficient. A good supply will have an efficiency of around 80 % at a certain load. Usually they will have their best efficiency when you load them at around 50% - 80% of their full load. The small DC adapters you have now will operate at this point.

Even the most effcient ATX power supply usually needs a load of at least 100 W or so to be as effiecient as those small adapters. ATX adapters need to be able to supply a lot of power up to 350 W for the smallest. This means compromises are made regarding the efficiency at 50 W for example. I bet that all your small adpters will not consume much more than that 50 W and that together thy are able to convert the mains power more efficiently than an ATX supply can.

So: not worth it, stick to what you have, it's better than you think it is.