I think your original post contains a few different elements that don't necessarily tie together.
The TTI Stage 2 tune will increase your engine performance. That's already installed, by the sound of it, so you're good there.
The larger 19 inch wheels could effect your handling positively or negatively, depending on what the setup is, but won't have impact on the engine performance...maybe overall vehicle performance, but that's about it.
The coilovers impact only the suspension, so it will effect handling. At a very extreme, the coilover setup cpuld effect the launching capability of the vehicle, but it's not like the IS's are rolling with 500+hp...so realistically the coilovers won't have any impact.

 

 

I think the answer to your question really depends on where you're driving the vehicle.
You could lower the car 2 inches and be driving in an area where the roads are good and there are little to no potholes, and you'd be perfectly fine.
Alternately, you could lower it 2 inches and be in a downtown core where the roads are terrible, and be bottoming out everywhere.
I'm not sure how the speed would really impact it bottoming out or not.
For sure lowering it an inch all around should be fine no matter where you are, but after that it's really environment and personal preference.

 

lol you're a funny guy, for Lexus IS XE30 chassis suspension ? how low can you go ? from 1 inch to 1.4 inch, if you lower than 1.8 inch on Lexus IS 3rd gen, you'll bottom out on every pothole and bump either City or Freeway with 2 persons +

You can have 19 inch rims but stay with OEM specs as close as possible or you'll be rubbing

Alternate your suspension or tires size indeed effects handling and performance, for example too big or too small tires specs, too small effects braking and handling, too big effects speed and gas mileage and definitely slow your car down

I think you're worrying about Power and Handling like me, but some people likes the nice looking, if you like perfect setup with 0 issue, here my personal reference :

With your Coilovers 1.2 inch drop in the front 1 inch drop in the rear. Wheel size 225/35/19 Offset +38 in the front . 255/30/19 offset +40 in the rear

 

 

im sure +38 front is perfect without issue but you can try +32 due to the least specs size that you'll have on it and the drop is minimal , i think it will be fine, as long as you have lesser than 1.8 drop and wheel specs like above.

 

 

You would still be fine with a +32 in front with a 225 tire.