I would defer to the manufacturers recommendation. Even with the itr cams its highly recommended to upgrade your valve springs to itr. However, many claim that if you arent revving past stock redline (assuming gsr head/valve train) you will be fine. Fine is true but fine basically means you should be good but theres always that risk factor. The bare minimum if you do upgrade is to get itr intake springs and move your gsr intake springs over to exhaust side.
Ive seen a couple sets of itr cams on craigslist recently so id browse there and forums. HMO can take a few weeks to get a new shipment in if they even get cams alone available. Havent seen them list cams in the used section in a long time. Have you?