IIRC the extra crew seats doesn't include the mass of the person sitting in the seat, just the seat. Infantry compartments include things like seats, straps, harnesses, and also the mass of the dudes and their equipment.
So while most of the time it wouldn't matter, with a small enough vehicle, a big enough unit, or a statistically greater rate of obesity among your customer base(assuming a large number of passenger seats), it might somewhat impact speed since the vehicle its normally rated weight.