I think the 'real-life' mechanic being represented by the abrupt boost is the redirection of energy from the shields to the engines, which seems pretty reasonable. A smooth curve would make it seem like what's controlling the speed (particularly when the shields are up) is something silly like air resistance, obviously not present in space. If modular engines were added, then having varying accelerations and tiers (or no tiers) would be a logical step, but for now the system seems pretty realistic and enjoyable. Frankly, the difference between a curve and constant seems completely negligible, since speed reaches its cap quite rapidly anyway.