First of all, belated eid mubarak to everyone!
Only use Toyota's T-IV for vitz transmission (not T-II which is used for corollas here). Btw, atf should be changed after like 40k kms or if there's something wrong in the fluid. If your atf is looking fine, then better change when u do 80k. Most probably it's been changed at 40k!
According to my experience, no need to use Hi Octane or mixture in vitz.
And I'm using ZIC A (not A+) - pretty satisfied by it. The fuel consumption has also decreased a bit. If the car was new brand, I might have used a more expensive oil - but as they say, once an engine is used to a particular oil... don't change it to something drastically different!
Dude, this is a common issue. Don't worry much. The speed ratios of auto trans are not the same as compared to a manual transmission (O/D is almost like the 5th gear in manual transmission). What I did notice was that after getting a new oil (Zic A) and filter, this issue was much reduced - so car was easily hitting 140 on level roads. And yes, fuel (cng or petrol) matters a lot at high speeds. My top so far has been almost 130 (on cng, after crossing 120 with o/d off) and 162 (on petrol) - then I stopped trying more!
Regarding tyres and wheels... I am with @storm on this. Manufacturer's recommended sizes are generally best for all purposes - unless u really know what you're doing. Low profiles will definitely be less comfy. And overall bigger sizes will definitely result in slower pick. Plus, the obvious problem of getting incorrect mileage and speed! And the more you load up your car (e.g. 4 or 5 ppl) u'll feel like towing a truck!