About 2-3 levels below in tests ATM. I would take his Aus 2015 exploits with a tinge of salt as not a single Aussie bowler managed to average below 30 in that series iirc. The only other time that happened was in 2004 but back then we had our greatest batting line-up in history. An excellent effort but not in the most challenging of conditions.
In ODI's , it is surely a contest. The way he was going between 2012-2014 was him matching SRT blow by blow for the title of the greatest Indian ODI batsman ever. But he will have to keep this up for a long time to surpass SRT.
Since we are talking of match-winners, we can use MotM award criterion (In ODI's the award is biased towards batsmen) Kohli has managed 21 MotM awards till date. Tendulkar managed to get 20 MotM awards till 1997 itself (these were against the bowlers of the calibre of Wasim,Waqar,Ambrose,Donald,Warne,Murali) , whereas it was between 1998-2003 that he really exploded winning a mind-boggling 29 MotM awards in 6 years! After that he had a phase between 2007-2011 when he was arguably the top ODI opener in the world and helped win dozens of games for India. SRT's longevity in ODI's is truly mind-boggling, but i would not put it past Kohli to match it. It will be difficult, but the way he has come back after a lean patch against a strong team in an important phase in the series, has really got my hopes up.
The MotM award criterion is not the be-all and end-all of gauging match-winners though (bowlers/allrounders get more in tests, batsmen get more in ODI's, players from better teams are likely to end up in the winning side and get more awards etc.)