^^^ are those masks and positions defined constants or variables? If they're variables the results will depend on their scope and qualifications and on the optimization level. Likewise you will get different (and variable depending on scope, qualification, and optimization) results with functions in place of macros. Either way, it's usually not worth stressing about one or two instructions per access, especially at this stage. You'll never create a generic approach that is maximally efficient in all situations, so get it working first, make it clear, clean, and maintainable second, and then, *if* you have *actual* concerns about performance, worry about those extra instruction, once you have a complete picture of the way the software needs to work and can optimize effectively.
Anyway the number of instructions per line of code is kind of meaningless without knowing which instructions and which platform, since not all instructions take the same number of cycles, and then it all goes out the window when you turn optimization on and the compiler is free to consolidate or reorder a lot of instructions.
As far as storing values between loop iterations, that depends on what sort of values you have to deal with. Just like when dealing with internal peripherals, a given register may be volatile or not, and 'const' or not, and those require different handling. If you're changing bits here and there across a number of registers and doing that a LOT, then sure, maybe a shadow register set makes sense--I did that for an OLED display recently for exactly that reason--but that's of limited value if those registers are volatile because you'll have to check those every time (or so) anyway. If that's the situation, then I would hope that the device has bitwise set/clear registers, otherwise that will get painful to deal with.
In any case, the SPI accesses will generally dominate the time it takes to deal with the peripheral, so your time is probably better spent making that as efficient as possible (DMA?) than worrying about whether it takes four instructions or five to access a particular shadow register.