Shimano cranks on BB30 bottom bracket
 

https://cimg4.ibsrv.net/gimg/bikefor...0823c51e64.jpg
I bought a cheap carbon frame (a Swift Ultravox TI for $125) and it came with this bottom bracket and originally had Sram Red cranks. What are my options for fitting a Shimano HollowTech crankset here as I have an Ultegra 6800 groupset? From what I've read, there are adapters or should I just knock out the existing pressfit bearings and get some 24mm ones?

You can get spacer adapters which use this bearing and make the hole 24mm and wider for the Shimano spindle. Or you can put in a bearing unit that sticks out the sides. Both work. I would base my choice on how the current bearings feel, the cost of replacement bearing units vs replacement bearings and the cost of the spacer adapter.

get the correct BB for your shimano spindle size...
the bearings will be larger, farther apart, and more durable

Does this frame have a 68mm shell then?

That's what BB30 technically is, but the information I was giving applies to 73mm and 86mm BB shells as well. I installed a lot of Shimano cranks in BBRight Cervelos using Wheels adapter inserts, and they held up very well.

The metal adapter BBs work fine, but occasionally fail to come apart properly when you want to service or replace them.