If the flight plan isd accepted by the IFPS then the route and the level are acceptable to ATC.
That is whole reason for having the IFPS in the first place.
Actually it's not the reason for having IFPS in the first place, it's a side benefit. The 'whole reason' is to provide an integrated and centralised processing system for the IFP Zone.
IFPS does not necessarily provide cast iron confirmation that a route is 100% correct or acceptable to ATC on the day. For example, from the IFPS User Manual:
The United Kingdom and Ireland's Standard Route Document (SRD) is
produced by NATS to assist Aircraft Operators in constructing RAD-compliant
route portions within UK and Irish airspace. The SRD is not a mandatory
document, and the routes contained within should be considered as
preferred routings only. The routes are promulgated to identify optimum
routings for operators with due regard to ATFCM requirements. The IFPS is
not obliged to comply with the SRD, and when processing messages, the IFPS
staff shall only validate messages in accordance with current and relevant
RAD restrictions.
There are also known problems with IFPS regarding operators filing on CDRs, for example.
It's a pretty good system and getting better but still has a bit of scope for improvement. Operators doing their homework and getting it right in the first place can greatly assist (not necessarily pointing the finger at the topic of this thread).
While one of the tasks of the IFPS is to provide flight planning assistance in finding correct routes where genuinely needed, message originators are also required to take responsibility for the quality of any messages they shall send to the IFPS for processing.
Of course ATC agencies can play their part too by ensuring that the mandatory use of routes are put in place by their national authority, and ensuring that IFPS subsequently has these contained in it's environment database.