No supercharger on an SE
15'* are gonna be tough with the L27. You're going to have to rip the guts out, port and polish, custom grind a cam, custom exhaust all the way back, custom intake, and find someone to burn you a chip. It'* not going to be easy.
For starters, do the intake, exhaust, 180° thermostat, and tranny cooler. After a full tuneup of course....plugs, wires, fuel filter, pcv, flush the trans, flush the coolant, etc.
The car will run much better at this point, with a good snappy throttle response. Might put you in the mid 16'* at that point. From there, you can decide if you're going to go for another second.