Saspeirs or Syspiritis from Greek sources is usually linked with historic Sper ( modern town Ispir in Erzerum province )
One explanation to this shift is based on the idea that the initial sa- is a Kartvelian plurial prefix. This theory has one serious problem. The supposedly singular form Sper is not seen anywhere. Not in Urartian, not in Greek and any other sources until Armenian times. More ever Georgian themselves use the form Speri. Without any prefixes. So how to understand this?
Well there can be another explanation to this shift. The Armenian language was quite intolerant to IE sibilants *s. They have undergone various changes, and today virtually nothing is left from initial sibilants. This is not specific to Armenian but has occurred in Greek and Iranian languages also. For example the European number six became hex in Greek while the Sanskrit sauma ( a drink ) became hauma in Iranian. In Iranian there was also secondary tendency to transform s into x.
In Greek and Iranian those shifts occured in early periods while in Armenian the exact date is unknown. But it is quite possible that it occured in post Urartian period.
Based on known phonetic shifts the term Syspir will sound in Armenian something like susper then will shift into husper. The second [s] will not become a [h] because it had consonant like p ( also t ) after him. This is a well known rule in Armenian.
But this form husper also will not last long and the initial hu- will drop and only Sper will remain. This dropping of initial laryngeals h is also well known.
Is there an evidence to prove this theory? Well there is. Strabo mentions a place known as Hyspiratis. Scholars believe that it is the same as the Syspiritis (=Sper). So why would Strabo use two different forms to denote a same place in the same text. Maybe he made an error. But maybe not. The difference between two mentions is the date. Syspiritis is mentioned when discussing old Jason's Argonauts period while the form Hyspiratis is mentioned in the younger context of Alexander Macedonian conquests. In most likelihood between this periods local Armenian speakers had transformed the term into Husper which is reflected in the Steabo's texts.
PS I discussed this subject with Martirosyan and he concurred with the plausibility of this theory. In fact it was suggested by him also. Simply I wasn't aware of that.