Ideally you want a little vacuum in there. The pcv normally does this. If you remove the pcv, and use a breather, then any pressure that builds up just goes out through the breather. You never see any vacuum then, if the pcv is removed. But you may get some blowby and oil coming out of the breather and getting on the engine or firewall.
The reason it affects the oil pressure, is because the oil is constantly escaping out of the bearings. If you had 40psi of oil pressure at regular atmospheric pressure inside the engine, for example, and then somehow you could pressurize the inside of the engine to 40psi, theoretically the oil couldn't get out because the pressures would be equal, with the air pressure trying to hold the oil back. But the oil pump would still be pumping, so the oil pressure would continue to build up until the oil started getting out again. And if you have a vacuum in the engine, it is trying to suck the oil right out of the bearings, so the oil pressure is less in that case.
The old rule of thumb for oil pressure was 10psi per 1000 rpms, to protect the bearings. I don't know if that rule bends a little for the LS1 engine or not... As long as your oil pressure is within factory specs, I'd say don't worry about it varying with or without the pcv connected.
(a plus of having a little vacuum in the crankcase, is less resistance seen by the backside of the pistons, and a little less air friction seen by the crank) At 4" away from crank centerline, when turning 6000 rpms, things are moving at over 209 feet per second, or 143 mph....
