In theory, there is no limit to how hard a topspin can be hit. The physics nature of topspin creates lower pressure below the ball and higher pressure above the ball...which is what makes a topspin drop. The more spin, the more this differential in pressure exists. A topspin can be hit harder and harder as long as the amount of critical spin is ample for the forward velocity of the ball.
Slice or underspin creates the opposite air pressure values. Thus, the harder a slice is hit, the more it will rise. There is a limit based on the velocity, the amount of underspin as well as the vector of the shot in which a slice can stay in.
So, no, the two shots don't have the same quantitative values for dropping into the court.
This should explain why the modern game has changed. Because the slice has such limitations, it technically has not changed over its use of say the last hundred years. However, the advent of topspin, with more severe grips and new perceptions of how hard a player can swing, has changed the topspin groundstroke considerably.
Is it no wonder that the number of 'chip and charge' players on tour have dwindled? The offensive nature of the topspin passing shot has increased dramatically whereas the slice approach shot has remained nearly constant over the last number of decades.
Hope this helps understand the two shots better!