I agree with your analysis cleardave in the sense that I'd rather have the card than not.
I also think that using boost in all the situations you describe does prove it's a powerful action.The example of Fel you used doesn't contribute to my point however, because obviously Fel gets boost for free.
I think that this card, maybe Stealth device given the recent discussions here, and marksmanship are the only mispriced cards in the whole game so far.
I disagree that it will be used a lot but I could be wrong. Certainly with Imperials, wouldl you rather put boost on a regular TIE, or just upgrade to an Interceptor? I'm speaking of competitive list building now, and not just building for fun.
Let's look at it this way: If you have only 4 points left to put on a Rebel list, would you take an Engine Modification, or would you rather have another card/cards? I think I would take it over a proton torp or a concussion missile personally, but not over R2D2 if that's an option, or even over R2F2 and any other 1 point card, or over Expose, or a Shield Upgrade, etc. etc.
I think you have to have a good reason to need to add boost at this cost. You could put Expert Handling on two ships and get added maneuverability along with loosing target locks.
Thanks to everyone for contributing to an interesting discussion so far.
The point I was making with Fel was more about the combo power of Boost and Push the Limit, with Fel being a good example. You can, however, use it with someone like Mauler Mithel, who can now Boost to move into Range 1, then drop Push the Limit to Barrel Roll out of your opponent's arc, Focus if you're already safe, or Evade if you're not. Is it the best way to use Mauler? I couldn't make that claim, but it certainly gives him a good amount of options to ensure he gets that Range 1 kill shot.
In that regard, I don't see a TIE Interceptor that really duplicates that for the same points;
Mauler with Push the Limit and Engine Upgrade is 24 points and is Pilot Skill 7
Saber Squadron TIE Interceptor with Push the Limit is 24 points and is Pilot Skill 4
I would take Mauler over the Saber Squadron on the strength of the higher Pilot Skill, personally, but there is something to be said for Saber's extra Attack dice at Range 2-3 that Mauler can't duplicate. So for the exact same points, you have two ships with similar qualities;
Boost Action, 4 Attack Dice at Range 1, but you're going to trade off attack versatility for Pilot Skill, and it comes back to personal preference. I couldn't objectively say which of the two is "better", but a personal choice would be to take Mauler, and thus the advantage of later action selection and earlier shooting priority. So in this case, yes, I would rather put the Boost on a regular TIE as opposed to taking another Interceptor.
This of course is talking about the ship in a vacuum and in a faster list of all Interceptors, maybe Mauler wouldn't be able to keep up, but if you're using a TIE Fighter heavy list, or wanting to get the most of Howlrunner support, it might make more sense to go that way.
On the Rebel side, I don't really see much use for Engine Upgrades outside of the examples I gave above, specifically with Wedge. I usually have some kind of Y-Wing presence in my list, and for 4 points, I would sooner take Torpedoes for Horton to exploit over an Engine Upgrade. The thing is, that's not to say anything negative about Engine Upgrade, or its points cost, just that for my specific purposes, I am not that excited about it in a Rebel list unless I attach it to a YT-1300.
Again, going back to the simple question asked in the thread title, I don't feel 4 points is too much for Engine Upgrade;
-It provides some decent flavour in "just for fun" matches without being over 5 points
-It provides some interesting TIE Fighter/Advanced alternatives
-The use of Boost is so versatile that the cost balances off attaching it to any existing craft in Wave 1 and Wave 2, and has the potential to be used well in the Wave 3 world and onward.
-There may be a Pilot card now or later that just gets ridiculous with Engine Upgrade that people may think that it becomes underpriced if some power creep comes into the game down the line.