Still amazes me how people spill this bullcrap without even looking at the specs. You're completely wrong.
i5:
http://www.cpubenchmark.net/cpu.php?cpu=Intel+Core+i5-4570+@+3.20GHz
i7 mobile:
http://www.cpubenchmark.net/cpu.php?cpu=Intel+Core+i7-3720QM+@+2.60GHz&id=895
Truth is, the i7 mobile is faster. I've experienced this as well, with an i7 15" MBP outperforming a 21.5" i5 iMac w/32GB RAM in Logic 9 (the iMac's CPU kept getting maxed out, whilst the MBP handled it like a dream).
The i7, although it's mobile, is a quad-core processor with hyper threading (8 virtual cores). The i5s are native quad-core without any virtual threads.
My advice: if you want the best performance, get a desktop i7 processor (i.e.: an i7 CPU in an iMac). Hope this helps.
🙂