Having gone the other direction, from the 345-19 Hoosier to the 335-19 Kumho, I can tell you, you are giving up a fair amount of rear grip. The 335 Kumho puts about a half inch less rubber on the ground per tire than the 345 Hoosier. Given the K-tire affiliation, I would recommend building around the 345-18 rear. Though I understand this car isn't an all-out dedicated autocross machine, so it probably doesn't matter, and the 19's do look better.
When you were on the KUMHO was it the new square shouldered tire? This thing has a huge cross section and the entire tread is on the ground edge to edge so I would be skeptical about any "real" advantages with the Hoosier as it's flex and roll over will change the actual contact patch more than the KUMHO does.
Since the car and I were new I thought that the 500# weight disadvantage would have had me much more than 1/2 second off the National caliber SSM drivers in attendance. Sure Eric broke and he told me what he thought was left on the table, but I will get better and so will the car so watch out,
I know the laws of physics, but look how fast the 320hp Cobra used to be lol.
Good luck this year and I look foreword to meeting up with you also.