Yes, your solution should work--I should have realized that the import mechanism would already create numbers for some of those things that are unambiguously interpreted as numbers. And as I was reading the first lines of your post I was already gong to write a solution like that for you. But to be more confident that something odd might slip through the generality of what you wrote (since it uses val_ which is a pattern that matches anything) I'd use,

GetMyNumber[val_?NumberQ] := val

I think that this should server you well. Let us know if there's anything that slips through...

Of course my function gives bizarre results in cases like this! So careful restriction to exactly the described cases is important:

In[5]:= GetMyNumber["5 is a number. But 6 is not"]

Out[5]= 5.6

And those cases cannot have either numbers or periods after the actual leading number.

I don't know if this is "elegant" but it certainly works because your numbers come first...

In[1]:= GetMyNumber[str_String] := 
 ToExpression[StringJoin@StringCases[str, DigitCharacter | "."]]

In[2]:= someNumbers = {"55", "12.7", "27%", "33.98%", "89", "91%abe"}

Out[2]= {"55", "12.7", "27%", "33.98%", "89", "91%abe"}

In[3]:= GetMyNumber /@ someNumbers

Out[3]= {55, 12.7, 27, 33.98, 89, 91}

Unfortunately something more elegant like using Interpreter["Number"] on your strings will not work since a number of the forms you have are not known number forms. Also, often using Interpreter[...] can be quite slow as it often uses the Cloud to do the interpretation.

But here is an example of the problem for a more general (semantic interpreter) case showing examples of an Interpreter working and not working on your examples:

In[4]:= Interpreter["InactiveSemanticExpression"] /@ someNumbers

Out[4]= {55, 12.7, Inactive[Quantity][27, "Percent"], 
 Inactive[Quantity][33.98, "Percent"], 89, Failure[
 "InterpretationFailure", 
Association[
  "MessageTemplate" :> MessageName[Interpreter, "semantic"], 
   "MessageParameters" -> Association["Input" -> "91%abe"], 
   "Input" -> "91%abe", "Type" -> "Expression"]]}

Which may not format correctly in this forum