So, i believe i've figured it out:
To get the acceleration of gravity and the lift of the balloon working together, we convert them both to forces (in newtons) and then just add/subtract and get the resultant force.
F = A
F = 25*-9.81
Gravity = -245.25 N
F = MA
F = 10*9.81 (9.81 because to counteract gravity it must be equal but opposite)
Lift = 98.1 N
Resultant Force = 98.1 - 245.25
Resultant Force = -147.15 Newtons (down, as indicated by the negative)
Then we can reverse that to get the resultant acceleration:
F = MA
-147.15 = 25A
A = -5.886 (m/s^2)
Then we use the SUVAT equations to get the time and displacement to peak.
S = Target (we're trying to figure out)
U = 5 m/s (stating Velocity)
V = 0 (no movement at peak)
A = -5.886 m/s^2 (we just figured this one out)
T = Target (another one)
V = AT + U
0 = 5.668*T + 5
5 = 5.668*T
T = 0.882145378 (seconds)
They do stay up longer (now i can sleep).
After i figure out if they go higher too, or just decelerate more slowly.
S = ((V + U)/2)*T
S = ((0 + 5)/2)*T
S = 2.5*T
S = 2.5*0.882145378
S = 2.20536345 (meters)
This is much higher than the original (without the balloon), which worries me i've done it wrong, but i can't see where.