If you follow the news and the forums you would know that arena attendance cap's out near 20k. You can build 40k arena but about 20k will be filled. Nomatter what you do. It was implemented to keep teams from building extremly big arenas -> thus making it impossible for someone to ever catch up.
Before the expansion,he has a 14k arena.When the work will be finished,he will have a 26k arenas,nearly the level of the cap
So,he can do this kind of expansion,It should be better if he makes it gradually,but he can do it
personnally i wouldn't use so much cheap places for reaching the cap, and more then 21.5k people are very ahrd to get - and this will hurt the income it is really superior to get close to the 20k from below not from above.
The problem is not to fill the places,but the fact that the bleachers are much more dipendant than the other sectors of the arena from the results.So,with a series ofloss,he will have few incomes
I did't wrote that he made the expansion in the right way,I only said that I don't think that with that kind of palace he will have problem with the arena cap,also if he will go beyond over 20k seats in the arena