1) is based on the fact that calling a DLL function is always using an extra indirect jump. Today, this is usually negligible. Inside the DLL there is some more overhead on i386 CPU's, because they can't generate position independent code. On amd64, jumps can be relative to the program counter, so this is a huge improvement.
2) This is correct. With optimizations guided by profiling you can usually win about 10-15 percent performance. Now that CPU speed has reached its limits it might be worth doing it.
I would add: (3) the linker can arrange functions in a more cache efficient grouping, so that expensive cache level misses are minimised. It also might especially effect the startup time of applications (based on results i have seen with the Sun C++ compiler)
And don't forget that with DLL's no dead code elimination can be performed. Depending on the language, the DLL code might not be optimally either. Virtual functions are always virtual because the compiler doesn't know whether a client is overwriting it.
For these reasons, in case there is no real need for DLL's, then just use static compilation.
EDIT (to answer the comment, by user underscore)
Here is a good resource about the position independent code problem
As explained x86 does not have them AFAIK for anything else then 15 bit jump ranges and not for unconditional jumps and calls. Thats why functions (from generators) having more then 32K have always been a problem and needed embedded trampolines.
But on popular x86 OS like Linux you just don't need to care if the SO/DLL file is not generated with the
-fpic (which enforces the use of the indirect jump tables). Because if you don't, the code is just fixed like a normal linker would relocate it. But while doing this it makes the code segment non shareable and it would need a full mapping of the code from disk into memory and touching it all before it can be used (emptying most of the caches, hitting TLB's) etc. There was a time when this was considered slow ... too slow.
So you would not have any benefit anymore.
I don't recall what OS (Solaris or FreeBSD) gave me problems with my Unix build system because I just wasn't doing this and wondered why it crashed until I applied