"overall" there is not an adjective but an adverb, as it can be assimilated to "in general" and can also be placed at the beginning or at the end of the clause without any change of meaning:
- Overall, our performance has been good.
- Our performance has been good, overall.
The writer probably decided to place it in mid position because there is "However" at the beginning and there is another clause after the one containing the word in question, so placing it at the beginning or at the end as is usually the case with the adverb "overall" would have clashed with the elements mentioned.
If it were an adjective, it would have been placed before the noun: Our overall performance has been good.
See, among others, these examples of the adverb "overall" after the noun here: http://www.ldoceonline.com/dictionary/overall
• There are more women in positions of power-though their numbers overall are still pitifully few.
• Six out of 10 Labour supporters, and 69 % of voters overall, opposed joining the euro.