First, a warning, this is going to get long. Because this isn't just off the top of my head "let's buff convoy" post, but rather one that takes into consideration a number of things.

There have been numerous posts on suggestions for boosting convoy reward, and the frequent rejection reason for those are:
1. Plunder is not as good as it first appear, 150 is not guaranteed.
2. Plunder is meant to be better to reward top players.

We shall examine each of those two points.

I must say that I accept #1 to be true, if we were to say the subject is ALL POTENTIAL PLUNDERS. That is to say, for example, that ranked 50 player in a server of 100-200 active player. Fact of the matter is, not every potential plunder choose to plunder, specifically because of this. If a player does not believe that they can get more out of plunder, on average, than convoy, then they would not choose to plunder in the first place. Fact of the matter is, those who choose to do plunder, especially the top 5-10 of each server, always come out far ahead. Therefore, there does, in fact, a large reward disparity between those who DO plunder (not those who CAN, but those who DO) and those who don't.
Therefore, #1 is a moot statement.

#1 does in fact, however, support #2. Those who are rewarded are in fact the top players. Now, I have nothing fundamentally against rewarding top players. Although it is already being done in just about every other competitive feature so one could question whether it is even needed in convoy/plunder, but that is not my point to argue and is a decision that is purely on the hand of the devs. So no, I will not argue against this reason and will accept it as it is.

However, I do have a problem with HOW the top player is being rewarded in this case--they are forced to perform a "crime" in order to receive their "rightful reward". This, frankly, is bad game design. Enticing players to perform an optional "bad" deed, especially one both in "name" (i.e. robbery is wrong) and one where it is ACTUALLY harmful for other player (the plundered receive reduced rewards) is corrosive to moral values. Now, I realize it is "just a game", but that doesn't mean it does not subconsciously suggest that wrongful acts can be rewarding, and this is why I do not like the "how" of the "reward top player" is being done.

And with that, here is my suggestion. I propose that we preserve "reward top player" while not make plunder the absolute superior choice, instead giving the top player two equally rewarding options in both convoy and plunder.
Since the current plunder only allows the top 50 to plunder, I will take that as the intended target for the rewards.
Convoy:
Top 10 player may do 4 SS convoy for free per day at 30 coupon each (or alternatively, 2 SS at 50/convoy)
11-20 can do 3 SS convoy for free per day at 30 each (or alternatively, 2 SS at 40/convoy)
21-50 can do 2 SS convoy for free per day at 30 each
Plunder
Top 10 player can plunder 3 times, at 50 coupon per SS success
11-20 can plunder 3 times, at 40 coupons per SS success
21-50 can plunder 3 times, at 30 coupon per SS success

As you can see, at every tier, plunder can theoretically get more reward overall, but as was stated, plunder is a risky business (granted, convoy could get robbed too)
This have a further benefit of increasing the amount of SS convoy being done in a server, thus providing more plunder targets, as currently the top reason why plunder don't get their full reward is actually the lack of target. Of course, any shift from plunder participation to convoy participation will further aid in this, which makes plunder better which cause some shift backward etc, until an equilibrium is reached, as opposed to the current state of going plunder regardless of lack of target because getting lucky on average once per day is better than convoy altogether.

At last but not least, by further bracketing, instead of just a broad of "top 50" into 3 tiers, it will encourage ranked battle participation. Let me tell you, ranked battle is dead. Our group, for example, can achieve 300 login in group activities, but struggles with 180 ranked battle per week, except when other event(such as this week's monthly) demands ranekd battle participation, when in theory, if 5 ranked battle is done by everyone that logs in, it should reach 1500(granted, some might be alts that doesn't much ever, but still, 500+ should be easy). That's how dead ranked battle is. Even this week it only barely passed 300, that's only a dozen player finishing the monthly event daily when I know for a fact we have more active players than that (almost 30 in GNW). Let me say it again, ranked battle is dead, if fighting for ranking carry a bit more weight with this, it can only be a good thing.