Weatherby freebore does reduce pressure. It does not increase velocity. A short throated Weatherby will usually be a little hot with factory loads.
The freebore may or may not affect accuracy. In general, if the freebore is within .0005" (one half of one thousandth) over bullet diameter, accuracy will still be good. Often more than good. The problem with the earlier Weatherbys was that the throat was often 3 or 4 tousandths over bullet diameter.
An illustration. I once chambered a 300 Weatherby for a friend and throated it conventionally. He was loading 200 grain Noslers and was able to obtain just slightly over 3000 fps before pressures became too high. He brought the rifle in and requested that I increase the freebore. I did so, extending the throat by 3/8 inch. The throat was cut at exactly .3080 diameter. The same load now produced about 2930fps. He increased the load to maximum and it topped out, once again, at just over 3000fps. Accuracy was unchanged and was very good. My current Weatherby reamers have throats which are Weatherby length but all are .0005" over nominal bullet diameter. Accuracy is excellent and factory loads are OK. Regards, Bill.