Motor savers makes a nice filter now:
http://www3.towerhobbies.com/cgi-bin/wti0001p?&I=LXVPA6&P=ML
They made one back in the day that had a tendency to let the end cap fall off and the filter element fall out. The new one is much better in that regard. They breath really well too.
For the pipe, I tried a similar resonator (single chamber) on my jato vs the stock plastic one and it is definitely louder and gives better power for the 2.5 or even my OS18Tm.
One that I tried that sounded really cool and gave great power on my revo (when I had it) was a fantom steel pipe. Sounded like a motocross bike. A nice "ding ding ding ding" sound when idling and a sharp tinny bark when on the gas.
I had this one:
https://www.ssl-stormerhobbies.com/cgi-bin/seekpart.pl?src=ns&pn=FAN20027
They make one for the maxx too:
https://www.ssl-stormerhobbies.com/cgi-bin/seekpart.pl?src=ns&pn=FAN20021
May have a hard time finding one though.
As for a bumper, I'd go with RC Solutions spring steel skid/bumpers. Much more durable.
http://www.horizonhobby.com/Products/Default.aspx?ProdID=RC+013
I ran those, but I put a rubber grommet behind them to limit how far they would bend and to absorb hard hits. With a dremel and some time, you can make it look not bad. I didn't spend that time....
If you use GH bulkhead braces, they have a nice hole in the middle that will allow you to put a bolt through and it will be strong, it's what I ran years ago:
Front:
http://www.ghhobby.com/proddetail.php?prod=2216B
Rear:
http://www.ghhobby.com/proddetail.php?prod=2217B
GH makes some really well made stuff for the maxx.
http://www.ghhobby.com/products.php?cat=5&pg=2
Here's what mine looked like (XTM 21 conversion chassis, but still a maxx at heart):
Was tough as nails with alloy bulks. I had literally hit concrete walls with that thing and it would just bounce back off. I have bad depth perception... doesn't work well when at a concrete skate park.
My only remaining issue was keeping diffs that lasted. Now with revo style diffs, that shouldn't be as much of a problem. I had this one 5 years ago. Pre-revo world...