I am getting the MSI B350 Tomahawk, but my current ram choices are not on the Mobo’s QVL.
Corsair - Vengeance LPX 8GB (2 x 4GB) DDR4-2400 Memory CMK8GX4M2A2400C16
Corsair Vengeance LPX 16GB DDR4 3000 C15 for DDR4 Systems - White CMK16GX4M2B3000C15W
How much will it affect my PC if I use these potential ram with my mobo?
QVL is what they can guarantee will work with that board, as is what they’ve tested. You have the best chance of compatibility by sticking to the memory kits that are on it. Some boards can be very picky about which sticks are in it for various reasons however in most cases, it works fine regardless.
Sticks that have a ‘slower speed’ variant of the same kit on the QVL have a good chance of working out. ie: 3000 mhz of a particular kit on it and you want 3200. It’s when you start to run kits that are not on it at all that you may run into problems.