Yes there's 3 different specs of 2.0 turbo 18i/20i/28i. Not sure if the OP meant to write 2.0 turbo or actually meant 20i
As Rob said any of the 2.0t will force you back in your seat earlier in the rev range than the non turbo 6's but in terms of outright straight line speed your 2.5Si would roughly be quicker than a 18i, on a par with the 20i and slower than 28i.
But with that low down torque if you jump in it for a quick go around the block in suburbia (low revs) even a 18i will probably feel more potent than yours. As you'll know, yours comes into its own when you open the taps. The E89 will feel more weighty though and less nimble than yours.