aerodynamically unstability and maneuverability

Soviet jets like MiG-29 and Su-27 doesn't have aerodynamically unstable designs yet is often said to be more maneuverable than their western counterparts,Is aerodynamic stability a big contributing factor to maneuverability?

RSS design for an aircraft means that one must go to an active flight control system to have controllable flight. The F-16 was the first RSS production fighter built, with others since. By going with an active system, control surfaces don't have to be as large to obtain the movement rates or displacements to make those same roll, pitch and yaw angles and rates. With an active system, split control movements are part of the control algorithms and they can be varied based on a huge number of factors.

Lots of data came out of the Grumman X-29 program in the late '80s for designing with inherently unstable characteristics.

as far as i remember diagrams both mentioned by you airplanes are statically unstable or with very narrow stability margin - this is why both of them are using active control system (this is not FBW but still active system)

I'm kinda going out on a limb here but we must differentiate between roll rate and turn rate. Turn rate and roll rate are independent. Accodring to America's Hundred Thousand, the P-61 was rated last in roll rate (it was big) but ranked third in turn rate (lots of wing). Stability affects roll rate and not necessarily turn rate. Instability affects roll rate, pitch rate, and yaw rate, depending on what you want to talk about. Roll rate is important in fighter for maneuvering, the other two for maintaining aircraft control. Roll instability allows a pilot to bank quickly therefore start a turn quicker, desirable. A complaint about the P-38 was that it rolled slower than the enemy, there fore got behind in the turn. The Focke-wulf 190 was noted for being a fast roller. Turn rate is a function of wing lift vs aircraft weight.

Too much instability can make an aircraft uncontrollable by a pilot. So that's where flight computers come into play. They react faster than a pilot and therefore the aircraft can be designed to higher instability standards allowing better maneuverability and still the pilot can maintain control.