Does use of memory with timing CL=10 instead of Apple used CL=11 slow my Mac down?
Yes, Crucial Ballistix Sport memory modules with CL=10 timing do work.

The results of an Xbench 1.3 "Memory Test":


Hynix CL=11: 618.25
Crucial CL=10: 699.12


The not by Crucial recommended memory modules perform 13% faster then Apple's factory default modules in this Mac.



Benchmarking performed on a 15" MacBook Pro (Mid 2012, MD103xx/A, MacBookPro9,1) with 2 modules of identical memory:


Apple default Hynix HMT325S6CFR8C-PB (PC3-12800S (DDR3-1600)
Crucial Ballistix Sport BLS2C8G3N18AES4CEU (PC3-14900 (DDR3-1866))




Detailed results:


Hynix 


Memory Test: 618.25
System: 679.54
Allocate: 2388.91 (8.77 Malloc/sec)
Fill: 414.93 (20174.73 MB/sec)
Copy: 630.47 (13022.12 MB/sec)
Stream: 567.11
Copy: 569.11 (11768.32 MB/sec)
Scale: 571.98 (11817.02 MB/sec)
Add: 568.58 (12112.06 MB/sec)
Triad: 558.29 (11943.20 MB/sec)

Crucial


Memory Test: 699.12
System: 699.53
Allocate: 2327.00 (8.55 Malloc/sec)
Fill: 435.50 (21174.97 MB/sec)
Copy: 639.94 (13217.72 MB/sec)
Stream: 698.71
Copy: 689.46 (14240.61 MB/sec)
Scale: 695.49 (14368.67 MB/sec)
Add: 716.26 (15257.79 MB/sec)
Triad: 694.24 (14851.47 MB/sec)