Convert ASCII Bytes from XML to VARCHAR
I think you should parse the text/Elements ,so that you can obtain the desired string.

SELECT 
 Tab.Col.value('id[1]', 'varchar(50)') AS Id,
 Tab.Col.value('text[1]', 'nvarchar(MAX)') AS TextAsNVarChar
,(SELECT CHAR(t.u.value('.','nvarchar(max)'))
        FROM Tab.Col.nodes('text/Element') AS t(u)
        FOR XML PATH('')
 )TextAsNVarChar_New
FROM @testxml.nodes('/ProgramWzgSignStates/WzgSignStates/Element') AS Tab(Col)


I used FOR XML PATH to obtain 2Hallo

Id                 TextAsNVarChar    TextAsNVarChar_New
AQ_A07_1_280_F3~ZA 507297108108111   2Hallo