A lot of that depends on bullet weight and powder choice. I would think that with heavy bullets 300gr+ you would see less velocity loss and you may want to go with a bit faster powder if you use small bullets.

Just a note Bullberry cuts the chambers very tight so you will most likely be stuck with using virgin brass or factory loads to start. Also unless you sent them a loaded round there will be very little freebore in their barrel. This could cause an issue with bullets like the 300gr Sierra.