It's not the T-5 that is notchy, it's the Tri-Ax. T-5s shift like butter and have a hell of a throw from the factory. Different shifters have different characteristics. I switched mine to the Tri-Ax despite it being a little notchy it gets me quick, short, and hard shifts. I hear the Pro 5.0 is not as bad, I love my Tri-Ax though. Check on the 5.0 forums about the different shifter characteristics.
I realized though that I never replied to the original posters question.
Modern driveline is a good shop, however the z spec is not "officially" rated at 450 hp. The only time I recommend a z spec for something over 400 hp is if someone absolutely cannot afford anything better. I also build z specs for $800 out the door, so it's an affordable option for most who are local to me.
however, I personally would never ever run a z spec or t5 with a motor 400 hp plus. The fact is, that it's too close to being topped out for what it can handle. You've proven through snapped main shafts and gears that a t5 is not enough for you. As well, the z spec re uses much of the same parts as a normal t5, including the main shaft and sliders and case.
In your case I would stay very far away from a z spec. I agree that while you need more gear, you also need a beefier trans too. You'll spend $1300 on a new t5z, break it, just to realize you spent half of what you already needed for a tremec.
with a tremec you wouldn't have to change the rear gear. First gear is lower than in the T-5z (3.27 vs. 2.95). That saves you some money, or at least it helps justify the initial cost of the tremec.
I have a tremec tko and 3.89 gears, and first gear is pretty useless with street tires. It takes off just fine in second....