large rails app starts up 2.7x faster in 1.9.3dev than 1.9.2p290

Given that 1.9.3.dev is effectively an “immature” 1.9.3 RC, I would say, yes, it will have a similar performance. However, I would not go using 1.9.3 at this stage as it has not yet been officially released.

Though what Ryan Bigg has said is true, Ruby 1.9.3 is and will stay
faster mainly due to the load.c performance patch. Here are a couple
articles explaining that:

http://www.rubyinside.com/ruby-1-9-3-preview-1-released-5229.html
http://www.rubyinside.com/ruby-1-9-3-faster-loading-times-require-4927.html