A throttle limiter could be a safety problem. There are times when you need all available power (which ain't much in a Mirage) to get out of someone's way. Or to get up a hill, even.
Just learn better throttle control. You opted for the manual transmission. Learn to drive it!
As I tell many of my driving students, a lot of driving is learning the "feel" of the car. To do what you're trying to do, you need to learn to "feel" the slightest amount of acceleration. If you want to keep the ECO light off and accelerate as efficiently as possible (and you're not in the way of other traffic!), you want to give it just enough throttle to barely accelerate. If you're used to goosing it and having fun feeling acceleration, this will be a drastic change for you. But, with practice and patience, you can do it.
However, there are those who will tell you (I've never tested it, but I somewhat believe them) that "moderate" acceleration (ECO light be damned) will still get you good fuel economy because you spend less TIME accelerating, even though you're accelerating a little harder. Just get up to speed, drop it in the highest appropriate gear and cruise.
I drove my Mirage the other day. Made about an 8 mile round trip with the AC on. Speeds typically 30-40 mph. Netted 52 mpg for the trip. No throttle limiter.
And I didn't even look at the ECO light. I just shifted at 1800-2000 rpm mostly, got as much DFCO as possible when slowing or stopping, did a little bit of neutral coasting here and there, and that was it.
Now, add in some typical city traffic and speeds more like 40-50... I'd be struggling to keep 48 mpg. And if I had to get on the highway and cruise anything over 55, it might dip down to as low as 42.
But, you don't need to mess with your car. The car is fine. It's capable of doing what you want it to do. You just need to learn to do it.
I could be persuaded to video one of my little drives for you if you think it would help. I've often thought about doing that, but didn't really think anyone would ever watch it if I did.