Nice ride! I definitely think the most economical way to get your power goal is LSx, and they're badass.
S54 can make those numbers too, with a similar badassary factor.
Just suggestions as you said you loved the smooth power band of your S4 (awesome ride ps). You can get a great tune with a SC and do well, but you won't get the same feel as an NA beast
All about taste
good luck in whatever route you choose, you'll have loads of support here!