Getting just a stiffer front sway bar will promote more understeer, because it will work the front tyres harder when they're already past their limit. You need to make the rear tyres resist more of the cornering force with a stiffer rear bar. Sways bars are often the best bang for your buck suspension mod, so IMO you should get adjustables for both front and rear, and play with the balance. If you're really tight, just get the rear bar.
I got the whitelines for my GTR, the front is on full soft and the rear full hard, to get as much understeer out as possible. This compliments the attessa tweaker which pushes more power to the front (so more tendency for understeer).