The two cars aren't actually available from the start even in the limited edition - they're only available early in the multiplayer mode. These cars can be found in the game world through jackspots even with the standard edition.
The bonus of "double speed points throughout their first four hours of multiplayer" is specific to the limited edition and will provide you a boost at the start of your multiplayer.
The other pre-order bonuses (the Speed, Strike, and Power packs) contain cars that again can be found in game, but with the added bonus of a couple of upgrades for each of these cars pre-unlocked. This isn't such a big deal as these can be unlocked by actually playing the game, and are normally gained through racing.
All in all, this means that the only truly 'exclusive' part of the limited edition is in fact the Satin Black livery. You get a headstart on cars and upgrades, and a bonus to speed points during the first hours of multiplayer, but the only part of the bonus that will stand the test of time is the livery.