mumbai-ahmedabad high speed train will use standard guage

See http://www.railwaygazette.com/news/single-view/view/indian-railways-to-order-25-high-speed-trains.html The japanese trains are standard gauge and incompatible with broad gauge used in India. According to https://en.wikipedia.org/wiki/Diamond_Quadrilateral possibly rest of high speed trains will still be broad gauge.

But it seems very unlikely that they will not reuse these tracks for Mumbai Delhi route. So yeah, all high speed routes, if done will use standard gauge. Ordinary long distance trains (which stop every hour or two) might be faster due to absence of Rajdhanis etc on same track but we are not developing broad gauge high speed trains for them.