HF18
I agree with all the remarks above regarding the high quality/uniformity and toughness of the Lapua brass.
The MV difference between the two will depend on barrel length, the desired bullet weights you'll be shooting, powders you will be shooting, and exactly how much improvement your chamber made to the Lapua case, and throat design of the reamers that will be used to chamber your rifle.
So the questions I'd ask are:
1. Barrel length
2. Desired bullet weights
3. COAL/bullet used on the dummy round used to design the reamer
4. Desired powders
While the performance of both chambers will be close, there is no substitute for case capacity when comparing two cartridges at equal pressures.
So it again depends on your goals for this rifle, and specifically the factors listed above.
There are Wby Improved designs that will put that case over 135+ grains of capacity after firing. You will have to run a Lapua Improved very hard pressure wise to make up for 10-12 grains of case capacity (depending on the amount of improvement done on the Lapua case) that a Wby Improved would offer.
If your barrel is very long, you favor the heaviest of bullet weights, the Lapua had a short throat, the Wby had a longer throat, and you don't mind shooting H50BMG, slower powders, the larger case capacity of the Wby will out perform the Lapua....at equal pressures.
But....at a cost of case life. Some will run the Lapua Improved at higher pressures and achieve the same velocities as the Wby, because the brass is so tough it allows multiple high pressure loadings over and over again.
Good luck.