SUM( A + B ) vs SUM(A) + SUM(B)
For the InnoDB/MySQL 5.6 table with 30M+ rows of FLOATs result is the next.

select SUM(x + y) from points;
/* Affected rows: 0  Found rows: 1  Warnings: 0  Duration for 1 query: 8,596 sec. */

select SUM(x + y) from points;
/* Affected rows: 0  Found rows: 1  Warnings: 0  Duration for 1 query: 8,799 sec. */

select SUM(x) + SUM(y) from points;
/* Affected rows: 0  Found rows: 1  Warnings: 0  Duration for 2 queries: 8,720 sec. */

select SUM(x) + SUM(y) from points;
/* Affected rows: 0  Found rows: 1  Warnings: 0  Duration for 2 queries: 8,580 sec. */

select SUM(x + y) from points;
/* Affected rows: 0  Found rows: 1  Warnings: 0  Duration for 1 query: 8,642 sec. */


In general the timings are of the same range so i suppose the difference is unsignificant.