No, angle of penetration matters in both games.
The only online article we could find covering this suggests that the angle matters in Portal 2, but did not in Portal 1, but after doing science, we can clearly demonstrate that the article's wrong - entry angle matters in both games.
Since you asked for undeniable proof, the following videos show you this mechanic in Portal 1.
4 tests on the lastest version of Portal 1
Tested on June the 6th, 2014 :
- Using a ball instead of Chell :
(If you are only going to watch one video, make it this one) I tested this on advanced chamber 17. The ball falls through the blue
portal and gain momentum (while falling). During the fall I place an
orange portal above the slanted surface, as soon as the ball comes
out of the orange portal I place another orange portal on the slanted
surface giving the ball an angle of penetration (45° I assume), the
ball is then ejected out of the blue portal but not vertically, it's
propulsed against the wall just in front of me. I repeated this
operation several times with different portal placement in order to
be sure that this was not due to a collision of some sort and it's
not, the outcome should be quite easy to reproduce if you want to
test it by yourself.
- Portal placed vertically : If speed vector was normal to the wall, I should
not pass through the emancipation grill
- Portal placed horizontally : I use the Angle of entry in the orange portal to land way further than the blue one.
To finish, in Portal 2 angle matters aswell but it's way easier to show it since its part of the gameplay with lasers.
Taking advantage of Angles of penetration in game
1. On the playground (a map designed with the purpose of learning those mechanics)
Example on vertical surface : In this example you can clearly see that the angle of penetration is conserved at the exit, allowing to reach the plateform on the left when exiting the portal.
Example on horizontal surface : In this example you enter twice the blue portal and goes out of the orange portal at 2 different angles (and speed but that's not the point here) allowing you to reach the top plateform. If the exit velocity vector was normal to the surface you wouldn't be able to reach the top plateform at all.
2. In an official chamber
This mechanic is for example what allows to finish the room 15 in 1 step in the least step challenge. Once you get into the "loop", you can place a blue portal on the ground and reach the elevator thanks to the momentum and the angle of exit (given by the angle of penetration). Again here, if the exit velocity vector was normal to the ground, when the blue portal was placed Chell should just have jumped vertically. Obviously it's not the case since she reaches the elevator.