Adding 'a' does make a difference.
Consider the following sentences:
I've got a little money.
I've got little money.
The first one means that you have a fair amount of money. Not much, but not very little either. However, removing the 'a' makes it the second sentence, which means that you have a less amount of money.
a little: a fair amount. Not much, but just enough.
little: almost none.
a few: a fair amount.
few: almost none.
"Activa i is just a little more than a smaller and cheaper Activa."
In this sentence, a little means 'a fair amount'. So it means that Activa i is a bit better than a 'smaller and cheaper Activa'.
However if you remove the 'a', you would get:
"Activa i is just little more than a smaller and cheaper Activa."
Which means that it is hardly better than 'a smaller and cheaper Activa'.
Both 'few' and 'little' on their own, are used in negative connotations. It gives a slight negative sound to the sentence.
And you are right; none of the sentences convey the intended meaning.