The Low Level Format wasn't recommended in the past. The hard disks released nowadays are fully compatible to low level format with the official tools released by the manufacturers. I have used Low Level Format for my Western Digital hard disk and everything was fine. The sectors were in their place, the capacity was assigned as full and as it was. The reallocated sectors come from places of the hard disk that are damaged. If you see that number increasing rapidly, then the hard disk could crash at any time.
There is no loss in capacity after a complete LLF. Also, keep in mind that LLF won't have an impact on the driver's capacity and performance. You'll practically restore it to factory condition.