I think this is a good idea for less than 4 concurrent user using a Atom 2701 based NAS if you are using it as a test database server for less than 4 concurrent users.
Atom is getting more and more popular in NAS. The dual core + HT architecture is capable to support 4 concurrent user. You may search for How Multi-Core Processors Accelerate your LAMP Applications to know why 4 concurrent user is good for Atom 2701 when you are using Apache with pre-fork.
Armada XP dual SoC is not commend because it may support only 2 concurrent session in the same configuration.
You might also need to consider about security for remote connection. Please read Using NAS as your Database Server to enable remote connection, grant privilege, and harden your NAS.
If you are not satisfied with the performance, you may launch memcached on NAS and load Memcache.so or Memcached.so on client to have your LAMP programs utilize database cache. Search for Optimize Synology DSM for ZurmoCRM for my test result from 34 to 10 seconds by the help of memcached on ZurmoCRM.
PHP accelerator will help, too. Please install them on your client. When I mentioned about client, I mean where your PHP program are interpreted or executed.
I have create a performance report using SugarCRM on QNAP TS-119PII and TS-269L. For single user, TS-119PII is better to be used as a database server than running SugarCRM. TS-269L seems no big different between them.
When there are more concurrent users, the difference between TS-119PII and TS-269L should become more noticeable. Based on the hardware architecture of D2701, 4 concurrent user is the best scenario for TS-269L.