'really instrumental' is correct and usable in written English.
You can use it to describe someone or something that had a significant role in bringing about a particular result. For example, "Mary's help was really instrumental in our success."
And I think David was really instrumental in that.
"Working out with Gary Roberts gave me that extra power and agility," Hodgson said, "and also working with our skating instructor, Dawn Braid — she's been really instrumental.
"The HBO addiction show was really instrumental in laying the groundwork, obviously with other activities, to educate the public," she said.
He was really instrumental in the park board in Williston, North Dakota, where my mom is from, in getting a pool built there so that other people would learn how to swim.
"It must have been really instrumental to the success of the group as a whole".
"OpenView, for example, has been really, really instrumental for the next stage of our journey".
It was Process of Weeding Out era, and he was really into instrumental music, so he made all the bands on his label do instrumentals for this record.
Serial technique is really an instrumental rather than a vocal idiom.
"I think the Americans With Disabilities Act has really been instrumental in helping us talk more openly about various disabilities and differences," he said.
"She and Christine Keeler were two teenagers who really were instrumental in focusing on the hypocrisy of [the Profumo affair], the sort of message which was being fed that there was one rule for the rich and another for the poor".
Peter Eyre - who also appears as a pale and reverberating Ghost, stretching out his hands to his son as if he truly misses him - makes the players really seem instrumental to the action.
