query overhead

Version.....
PostgreSQL 9.1.2 on x86_64-unknown-linux-gnu, compiled by gcc (GCC) 4.5.2, 64-bit

Server.....
Server: RX800 S2 (8 x Xeon 7040 3GHz dual-core processors, 32GB memory
O/S: SLES11 SP1 64-bit

Scenario.....
Legacy application with bespoke but very efficient interface to its persistent data. We're looking to replace the application and use
PostgreSQL to hold the data. Performance measures on the legacy application on the same server shows that it can perform a particular read operation in ~215 microseconds (averaged) which includes processing the request and getting the result out.

Question......
I've written an Immutable stored procedure that takes no parameters and returns a fixed value to try and determine the round trip overhead of a query to PostgreSQL. Call to sp is made using libpq. We're all local and using UNIX domain sockets.

Client measures are suggesting ~150-200 microseconds to call sp and get the answer back

ping to loopback returns in ~20 microseconds (I assume domain sockets are equivalent).

strace of server process I think confirms time at server to be ~150-200 microsecs. For example:
11:17:50.109936 recvfrom(6, "P\0\0\0'\0SELECT * FROM sp_select_no"..., 8192, 0, NULL, NULL) = 77 <0.000018>
11:17:50.110098 sendto(6, "1\0\0\0\0042\0\0\0\4T\0\0\0(\0\1sp_select_no_op"..., 86, 0, NULL, 0) = 86 <0.000034>

So it looks like a local no-op overhead of at least 150 microseconds which would leave us struggling.
Could someone please let me know if this is usual and if so where the time's spent?
Short of getting a faster server, is there anything I can do to influence this?

Thanks,
Andy