Well base model wrx while heavy can be modded to surpass STI stats. While not too cost efficient. Wrx + Lots of $$ parts and tune lots of $$. Power to weight when compared to mz3 or ms3 weight is comparable.
Want a beast?
Put a GM 6.0L into a miata (95+)
Or why not do a MZ3/MS3 V8 swap. With the aftermarket for these GM small blocks and the durability. VS the V8 ford(BOO!) focus swap.
I mean **PooP** a 5.3 GM V8 will run for 150K+ with a Whipple supercharger and produce + 400hp at the wheels. whipple kit 3500$ engine + tranny 1500$ (ebay).
Get the mounts and cross members fabricated. Then use a Caddy CTSV rear end. Still total cost less than the JDM or US SUBARU swap plus the tune. SUBAURU STI's do not like aftermarket, besides working the symmetrical All wheel drive To fit.
BTW I have a not so stock 08 MZ3 GT every option known to man( DEBADGED( besides AXELA). not in it for any Et's, light to light or 1/4 times love the fuel economy and peppy feeling ZOOM ZOOM. ALSO 1999 CHEVY 2 DOOR TAHOE 5.7 V8 290 ish HP 360 TQ Stock internals beside CRANE Comp Cam Nice Holley Regulated, External Tranny controler.( soon to be a predator tranny).
I know power and with the MZ3 Blowing up after 300+ hp just go like flying miata V8 RWD INSTANT POWER.
If you want a beast 4 cylinder get a SRT4 NEON and build it up.
OR NEW cobalt coupe SS TURBO or TURBO SKY ( I have no clue how tuneable they will be)
Awesome Machines LIGHT AND GM