Steps 1 and 2 are required so it enables you to raise your amp gains after level matching, if required/suitable. It isn't suitable for you as your centre eill be attenuated. And even though the ms-8 will limit the power of amped channels, its still much better for you to level match everything before, you'll get a better autotune.
So you've got 3 L7 mids across the front, and you're amping two of them. So you're going to have to either somehow get a much more efficient centre to match the SPL level of your other 2 L7 mids on ~ 150w of power. Maybe a kappa infinity 42.4? If you're after SQ this won't really be a good idea though. Or you could keep your L7 centre and upgrade to Morels, which are slightly less efficient and also 4 ohm, so they might even match your centre even on 150w. There are online calculators to estimate this. Have a play with different combinations and see what works. You can buy Morels for £230, the extra £50 definately worth it.
I'd measure the SPL levels from the driver head position.
2008 E92 335D SE - Ohlins
2010 E90 320D SE
1994 E36 318I SE
1996 E36 316I SE