The only perf advantage to running IIS on 64bit vevrsus 32-bit is to allow access to a much larger memory address space.
If you are doing normal ASPX page processing, then it’s likely you don’t need to address more than 4gb from any single process. Suppose you run in 32-bit mode with a web-garden with multiple worker processes on the same machine. In that case each process can address up to 4gb.
The big advantage can come when you perform caching. A 64-bit process can maintain a huge in-memory cache (assuming you have the 32GB or more of RAM to support it) to allow you to cache complex page content or data, on the web server. This allows perf gains when the data is more expensive to generate than it is to retrieve – for example if the data is an elaborated form (let’s say the result of a monte carlo simulation), or if the data resides off-box and the network IO time is much more expensive than cache-retrieval time.
If you do not use caching, then 64-bit IIS is not going to help you. It will require 64-bit pointers for every lookup, which will make everything a little slower.
64-bit servers are much more effective when used for databases like SQL Server, or other data management servers (let’s say, an enterprise email server like Exchange), than for processing servers, such as IIS or the worker processes it manages. With a 64-bit address space, servers that need to manage data can keep much more of that data in memory, along with indexes and other caches. This saves disk IO time and elaboration time when a query comes in. Most Web apps don’t need to address more than 4gb from a single process.
Maybe a useful analogy: In transport, an large SUV is like a 64-bit machine, while a regular, compact passenger car is like a 32-bit server. You can carry much more stuff in a large SUV, and it has a larger towing capacity, seating for 8 people, and a GVWR of 8600 lbs. But with all that, you pay. The truck is heavier. It uses more fuel. If you are only carting around 2 people and one duffel bag, you don’t need an SUV. You’ll be better off with the smaller vehicle. It can be speedier and more efficient.