GWT performance optimization singleton classes, RPC types, server round-trips

Using GWT 2.5, I am currently going through performance optimizations of a reasonable size GWT client application which is served by C# .NET RPC interfaces on the server side. I have been through the Google I/O video and we have made significant impovements by reducing number of widgets and third party library dependencies, introducing split points and reducing round trips etc. There are a few area that I am not sure of that I would like your views on.

1. I have a buttons toolbar (using our custom button which extends GWT Button) which is frequently redrawn based on the state of the aplication and there are about 150 buttons to select from to add/remove to the toolbar. Currently I have a singleton class per button which extends my custom base button class. How does this translates into JS and whether using singleton classes like this have any impact on the application size and performance? What happens when a singleton button is removed from parent and re-added? Would it stay in memory from the first instantiation for the life time of the app?
2. As I am using .NET on the server side, I am using a simple open source third-party JSON-RPC library to make calls to the server. Is there any thing I can do for example exclude GWT-RPC implementation from the JS to reduce the size? Would blacklisting *.* make any difference?
3. One of the things mentioned in the GWT performance videos was to determine the user-agent on the server and dispatch the appropriate JS to avoid a round trip. However I can not figure out how to compile the application so that I can know which .cache JS file belongs to which browser implemntation unless I compile it for each user-agent one at a time?

thanks

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