To the specific question of gifted assessment, I would say that there is very little risk to proceeding with further assessment. I tend to take the position that more information is better.
It seems to me, though, that your larger question is, is he really gifted, or "just" hyperlexic? We've had discussions about this before on this board. I will reiterate that the key distinction between hyperlexia as a "disorder" and early reading skills in the gifted context is comprehension.
Decoding does take some cognition, but not as much as you would think, especially for someone who does not have particular obstacles in one of the three pillars of reading decoding (phonological awareness, phonological memory, rapid naming/automaticity). I have personally evaluated multiple students with IQs below 70 (some well below) who could decode (read) and encode (spell) at an entirely age-appropriate level of accuracy and fluency (adult-level, for the secondary-age students). None of them, however, could comprehend at a level beyond their cognition, especially when asked to make inferences. We also expect the average eight or nine year old to be able to master the entire range of phonetic decoding skills. This tells you something about the level of cognition necessary to master phonetic decoding--it's not that high. It also explains the old teacher's tale that early readers will "even out" by third grade. It's not that truly advanced readers level off, it's that the ceiling on phonetic decoding skills is only about that high. And some of the other early readers were really early decoders, not early comprehenders.
There are also multiple pathways to early reading, some of which are not fundamentally language-based. (It's often more visual than verbal.) So that an early reader may actually be gifted in a visual-spatial or pattern-recognition area, rather than language itself.
Then there is the third question of twice exceptionality, which, given his history, is probably an area that deserves additional attention, and possibly evaluation. If no concerns are identified, then maybe that is reassuring. If they are, then that gives you something to focus on for remediation.
