The Islanders-Penguins first-round playoff series features John Tavares and Sidney Crosby, two of the league’s elite centers, as well as a host of prolific scorers, from Matt Moulson to Evgeni Malkin and Jarome Iginla. But Kyle Okposo has been as effective as any of them, right up there with Crosby. Yes, the same Kyle Okposo who was a target of boos from disgruntled Islanders fans early in the season, who managed just four goals and 20 assists during the regular season, who some were beginning to think wasn’t a top-six forward. “Obviously, I had a tough year statistically for myself, but I just wanted to keep going,” he said after the Isles’ hard-to-swallow, 5-4 overtime loss dropped them into a 2-1 hole in the series. “I know how I can play and I know I had played better during the year. I just wanted to come out and prove it in this series.” Through three games, he has proven plenty — that his strong finish to the regular season wasn’t a fluke, that he has found himself and his game. Okposo’s renaissance continued in yesterday’s loss, as the physical fifth-year pro keyed another Islanders comeback, scoring short-handed on a breakaway — his second goal of the series — and was clearly the team’s best forward. “You can just see he’s taken it to another level,” Tavares said. Said Okposo: “A month a half ago, I just started to play better, be more effective. I feel like I was having more of an impact on the game.”