
IMO, IR signature is practically impossible to guess. There are features in F-35 and F135 that could make it very possibly have lower IR signature.
1. Higher bypass ratio means there is more cool air available to cool the engine and exhaust
2. Longer length meaning that hot parts of engine are further away from exhaust giving somewhat more time and space to cool the exhaust gases and even out the heat
3. Being developed directly from F119, it's likely there are lessons learned from it and improved in F135. This includes also IR signature.
IMO, F-35 very likely has lower IR signature flying subsonic due to these factors and F-22 very likely has lower IR signature supersonic due to supercruise ability.