Fix for parameters used mostly for pre-fetching egamma chains

Tagging @wiedenma as we discussed this yesterday. We need to better set the RoI to be (pre-fetched). @dbakshig could help spreading the word in egamma (in principle, no change of any value should happen, but a slightly improvement in the pre-fetching should be observable). @rbielski should also be marked here, as he indicated me the portions of code where these things change. @tamartin might want to check, later on, possible cost changes.