The poster is a poster... it looks nice.
To continue on with the shadow, why are the insides of letter's shadows treated differently ??
how so? The main text has a uniform inner shadow effect applied. Is that what you're referring to?
This.............
Besides, when is a shadow lighter then the letter ?? Shadows are cast from a light source, hence the word shadow..... unless your letters are jet black on a very light background, but they'll still be darker than the background color. Otherwise, it's not a shadow.
A shadow would not be the place to try and get creative to achieve solid impressions. Either re-worked the letter itself or like OP said, let well enough alone.
If you want to give the impression of something strong.... have sun rays or something shining through. That's always gives a strong feelings.