Studs are a good solution because more of the torque that you apply to the nuts goes into actual clamping force on the head. Since studs do not turn in the block threads, there is no loss of load due to friction down there. Any steel used in a head bolt is going to be harder than the aluminum in a block thread. There is a limited number of times that the head bolts can be torqued onto an aluminum block before the threads start to fail. Probably about 10 times before you start pulling threads out of the upper part of the bolt holes, then it's all downhill from there.
Adding fasteners is good, but selection of the location for these fasteners is very difficult in an engine that has intake ports, exhaust ports, water passages, and oil passages. If you do find a place on the cylinder head deck to put more bolts, chances are that you will create water leaks.
Drilling and tapping for bigger bolts is a good way to get more clamping force, but this may be unsuccessful if there is not enough boss material in the head and block to accommodate the larger bolts.