We would be in near future implementing a solution to modernize our iSeries applications 
written as RPG programs with some stored procedures, and our preferred way is leveraging the latest and greatest of what Java has to offer in this space.

From googling and checking other questions here on STOVFlow, JTOpen seems to be the defacto
library/toolset which has worked for most and I was encouraged to see that Tomcat runs on an I-series box with out any issues. 

With this as the background, I am thinking of the following as the high level sol arch

1. Install IBM JRE and use JTOpen's capabilities to invoke RPG Programs and in some cases directly call the stored procedures running on DB2
2. Have Tomcat host a modern web application built using Grails and other frameworks (Camel, Smooks) to provide an application logic layer which would fill any mediations, transformations required for the old functionality to be offered to the user from a browser

Questions-

1. If any one of you has been involved in such an exercise, please share the pitfalls with this approach
2. Is there a significant performance drop with respect to response times for the end user?
3. Would it be better to some how expose the JT400 code as web services and run the web app on a different machine altogether consuming these web services?