hi Julio,
SAP Portal is still on the picture in many customers I visit.. SAP is doing a big push into ABAP related stack and have been neglecting the Java stack.. so in a large enough time-frame, my assumption is that you will either run Java in TomEE (HCP) or you will end up in a total SAP vendor lock-in using ABAP & HANA.
About your question, SAP Portal requires a J2EE AS behind, and you often have one to use for your developments. Considering how SAP really uses J2EE, this means little.. but then again, you're not limited to run "SAP related" in the AS.
The very first question you should ask yourself is, why on earth you would choose OData over JAX-RS2? - I understand SAP push because they are developing a lot of stuff to be extended and customized, so it's good to have some standards in place (make easier to understand) - but on the other hand, OData also brings you limitations on how to solve your issues.. this is the first step, analyse if you need OData because if you don't, Jersey will always out-perform OData (olingo) and provide you more flexibility.
Considering how you want to do this, exposing an application from the Portal is no different than exposing any other application for what it matters. You could code all your "service" into an enterprise application and deploy it - Case you need to access the ABAP layer from Java, the options are sort of limited: pretty much ARFC using a CAF/ejb or perhaps you can use SREST in ABAP.. but then again, that could defeat a little the purpose of having the service in the Java layer.
As of Fiori and UI5, there are many options - there's really nothing to setup other than usual, you import the UI5 SDA's into your track and use them - they pretty much expose a centralized "resource" system for UI5, you don't even need to use it - you can pretty much develop the application on Sublime, VIM, Eclipse, WebIDE.. whatever, just make sure you work on your "DC" workspace (this is a web-archive DC) so you can easily generate builds to bundle and deploy into the J2EE.
Also, even with very little attention from SAP the J2EE+JAXRS+CAF+RFC has always been faster than ABAP+ODATA/GW on jMeters (total load) and response time (go figure, OData in GW is bloatware as far I'm concerned) - however, it seems that in the "SAP space" Java developers are rare in comparison to ABAP developers, so most companies choose for what Christopher said: - you follow what SAP tells you, you don't question performance and you throw more and more bucks on license every time they decide to "launch something new" that you can easily solve using Java and Open Source.
Hope it helps,
Dan.